Download Freshwater Algae: Identification and Use as Bioindicators

1
Introduction to Freshwater Algae
1.1
General introduction
Algae are widely present in freshwater environments,
such as lakes and rivers, where they are typically
present as micro-organisms – visible only with the aid
of a light microscope. Although relatively inconspicuous, they have a major importance in the freshwater
environment, both in terms of fundamental ecology
and in relation to human use of natural resources.
This book considers the diversity of algae in freshwater environments and gives a general overview of
the major groups of these organisms (Chapter 1),
methods of collection and enumeration (Chapter 2)
and keys to algal groups and major genera (Chapter 4). Algae are considered as indicators of environmental conditions (bioindicators) in terms of
individual species (Chapter 1) and as communities
(Chapter 3).
1.1.1
Algae – an overview
The word ‘algae’ originates from the Latin word for
seaweed and is now applied to a broad assemblage
of organisms that can be defined both in terms of
morphology and general physiology. They are simple
organisms, without differentiation into roots, stems
and leaves – and their sexual organs are not enclosed within protective coverings. In terms of physiology, they are fundamentally autotrophic (obtaining
all their materials from inorganic sources) and phoFreshwater Algae: Identification and Use as Bioindicators
C 2010 John Wiley & Sons, Ltd
tosynthetic – generating complex carbon compounds
from carbon dioxide and light energy. Some algae
have become secondarily heterotrophic, taking up
complex organic molecules by organotrophy or heterotrophy (Tuchman, 1996), but still retaining fundamental genetic affinities with their photosynthetic
relatives (Pfandl et al., 2009).
The term ‘algae’ (singular alga) is not strictly a
taxonomic term but is used as an inclusive label for
a number of different phyla that fit the broad description noted above. These organisms include both
prokaryotes (Section 1.3, cells lacking a membranebound nucleus) and eukaryotes (cells with a nucleus
plus typical membrane-bound organelles).
Humans have long made use of algal species, both
living and dead. Fossil algal diatomite deposits, for
example, in the form of light but strong rocks, have
been used as building materials and filtration media in
water purification and swimming pools. Some fossil
algae, such as Botryococcus, can give rise to oil-rich
deposits. Certain species of green algae are cultivated
for the purpose of extracting key biochemicals for
use in medicine and cosmetics. Even blue-green algae, often regarded as nuisance organisms, may have
beneficial uses. This is particularly the case for Spirulina, which was harvested by the Aztecs of Mexico
and is still used by the people around Lake Chad as
a dietary supplement. Spirulina tablets may still be
obtained in some health food shops. Blue-green algae are, however, are better known in the freshwater
environment as nuisance organisms, forming dense
blooms having adverse effects on human activities
Edward G. Bellinger and David C. Sigee
2
1 INTRODUCTION TO FRESHWATER ALGAE
by producing toxins, clogging water courses and impairing recreational activities.
1.1.2
Algae as primary producers
As fixers of carbon and generators of biomass, algae
are one of three major groups of photosynthetic organism within the freshwater environment. They are
distinguished from higher plants (macrophytes) in
terms of size and taxonomy, and from photosynthetic
bacteria in terms of their biochemistry. Unlike algae,
photosynthetic bacteria are strict anaerobes and do
not evolve oxygen as part of the photosynthetic
process.
The level of primary production by algae in freshwater bodies can be measured as fixed carbon per
10−2
0
1
2
3
4
5
Figure 1.1 Algal photosynthesis:
primary production in six contrasting lakes. Showing the rate of photosynthesis (carbon fixation per unit
area) with depth in lakes of varying nutrient availability. These range
from very eutrophic (Lake George,
Uganda) to eutrophic (Clear Lake,
California; part of Lake Baikal),
mesotrophic (Castle Lake, California), oligotrophic (Lake Tahoe,
California–Nevada) and very oligotrophic (Lake Vanda, Antarctica).
Reproduced, with permission, from
Horne and Goldman (1994).
Depth, m
10
unit area with time (mg C m−3 h−1 ), and varies greatly
from one environment to another. This is seen, for example, in different lakes – where primary production
varies with trophic status and with depth in the water
column (Fig. 1.1). Eutrophic lakes, containing high
levels of available nitrogen and phosphorus, have very
high levels of productivity in surface waters, decreasing rapidly with depth due to light absorption by algal
biomass. In contrast, mesotrophic and oligotrophic
lakes have lower overall productivity – but this extends deep into the water column due to greater light
penetration.
Although algae are fundamentally autotrophic
(photosynthetic), some species have become secondarily heterotrophic – obtaining complex organic
compounds by absorption over their outer surface or
by active ingestion of particulate material. Although
10−1
Carbon fixed, mg C m−3 h−1
1
10
Very eutrophic
Lake George,
Uganda
Eutrophic
Clear Lake
California
Oligotrophic
Lake Tahoe,
California − Nevada
Very
oligotrophic
Lake Vanda,
Antarctica
102
Mesotrophic
Castle Lake,
California
Eutrophic part
of Lake Baikal
15
20
30
40
50
60
Approximate areal
values, mg C m−2 h−1
George
Clear
Castle
Baikal
Tahoe
Vanda
380
116
70
60
9
0.6
103
1.1 GENERAL INTRODUCTION
such organisms often superficially resemble protozoa in terms of their lack of chlorophyll, vigorous
motility and active ingestion of organic material, they
may still be regarded as algae due to their phylogenetic affinities.
1.1.3
Freshwater environments
Aquatic biology can be divided into two major disciplines – limnology (water bodies within continental
boundaries) and oceanography (dealing with oceans
and seas, occurring between continents). This book
focuses on aquatic algae present within continental boundaries, where water is typically fresh (nonsaline) and where water bodies are of two main types:
r standing
(lentic) waters – particularly lakes and
wetlands
r running
(lotic) waters – including streams and
rivers.
The distinction between lentic and lotic systems
is not absolute, since many ‘standing waters’ such
as lakes have a small but continuous flow-through of
water, and many large rivers have a relatively low
rate of flow at certain times of year. Although the
difference between standing and running waters is
not absolute, it is an important distinction in relation
to the algae present, since lentic systems are typically
dominated by planktonic algae and lotic systems by
benthic organisms.
Although this volume deals primarily with algae present within ‘conventional freshwater systems’
such as lakes and rivers, it also considers algae present
within more extreme freshwater environments such
as hot springs, algae present in semi-saline (brackish)
and saline conditions (e.g. estuaries and saline lakes)
and algae present within snow (where the water is
in a frozen state for most of the year).
3
(largely benthic) organisms. Planktonic algae drift
freely within the main body of water (with some
species able to regulate their position within the water
column), while substrate-associated organisms are either fixed in position (attached) or have limited movement in relation to their substrate. These substrateassociated algae are in dynamic equilibrium with
planktonic organisms (see Fig. 2.1), with the balance
depending on two main factors – the depth of water
and the rate of water flow. Build-up of phytoplankton populations requires a low rate of flow (otherwise
they flush out of the system) and adequate light levels,
so they tend to predominate at the surface of lakes and
slow moving rivers. Benthic algae require adequate
light (shallow waters) and can tolerate high rates of
water flow, so predominate over phytoplankton in
fast flowing rivers and streams. Benthic algae also
require adequate attachment sites – which include inorganic substrate, submerged water plants and emergent water plants at the edge of the water body. The
distinction between planktonic and non-planktonic
algae is ecologically important and is also relevant to algal sampling and enumeration procedures
(Chapter 2).
Planktonic algae
Planktonic algae dominate the main water body of
standing waters, occurring as a defined seasonal succession of species in temperate lakes. The temporal sequence depends on lake trophic status (Section 3.2.3; Table 3.3) with algae forming dense
blooms (see Glossary) in eutrophic lakes of diatoms
(Fig. 1.16), colonial blue-green algae (Fig. 1.5) and
late populations of dinoflagellates (Fig. 1.10). During
the annual cycle, phytoplankton blooms correspond
to peaks in algal biovolume and chlorophyll-a concentration, and troughs in turbidity (see Fig. 2.8).
Benthic algae
1.1.4
Planktonic and benthic algae
Within freshwater ecosystems, algae occur either
as free-floating (planktonic) or substrate-associated
Benthic algae occur at the bottom of the water column in lakes and rivers, and are directly associated
with sediments – including rocks, mud and organic
4
1 INTRODUCTION TO FRESHWATER ALGAE
Table 1.1 Size Range of Phytoplankton
Category
Linear Size (Cell or
Colony Diameter, µm)
Biovolume* (µm3 )
Unicellular Organisms
Colonial Organisms
Picoplankton
0.2–2
4.2 × 10−3 –4.2
–
Nanoplankton
2–20
4.2–4.2 × 103
Microplankton
20–200
4.2 × 103 –4.2 × 106
Macroplankton
>200
>4.2 × 106
Photosynthetic bacteria
Blue-green algae
Synechococcus
Synechocystis
Blue-green algae
Cryptophytes
Cryptomonas
Rhodomonas
Dinoflagellates
Ceratium
Peridinium
–
Diatoms
Asterionella
Blue-green algae
Anabaena
Microcystis
Biovolume values are based on a sphere (volume = 4 /3 π r3 ).
Table reproduced from Sigee, 2004.
debris. These attached algae may form major
growths on inorganic surfaces or on organic debris,
where they are frequently present in mixed biofilms
(with bacteria, fungi and invertebrates also present).
Under high light conditions, the biofilm may become
dominated by extensive growths of filamentous algae – forming a periphyton community (Fig. 2.23).
Attached algae may also be fixed to living organisms
as epiphytes – including higher plants (Fig. 2.29),
larger attached algae (Fig. 2.28) and large planktonic colonial algae. Some substrate-associated algae are not attached, but are able to move across
substrate surfaces (e.g. pennate diatoms), are loosely
retained with gelatinous biofilms or are held within
the tangled filamentous threads of mature periphyton
biofilms.
Many algal species have both planktonic and benthic stages in their life cycle. In some cases they
develop as actively photosynthetic benthic organisms, which subsequently detach and become planktonic. In other cases the alga spends most of its actively photosynthetic growth phase in the planktonic
environment, but overwinters as a dormant metabolically inactive phase. Light micrographs of the distinctive overwintering phases of two major bloomforming algae (Ceratium and Anabaena) are shown
in Fig. 2.7.
1.1.5
Size and shape
Size range
The microscopic nature of freshwater algae tends
to give the impression that they all occur within a
broadly similar size range. This is not the case with
either free floating or attached algae.
In the planktonic environment (Table 1.1), algae
range from small prokaryotic unicells (diameter
<1 µm) to large globular colonies of blue-green algae
such as Microcystis (diameter reaching 2000 µm) –
just visible to the naked eye. This enormous size
range represents four orders of magnitude on a linear
basis (×12 as volume) and is similar to that seen
for higher plants in terrestrial ecosystems such as
tropical rainforest.
Planktonic algae are frequently characterized in
relation to discrete size bands – picoplankton
(<2 µm), nanoplankton (2–20 µm), microplankton
(20–200 µm) and macroplankton (>200 µm). Each
size band is characterized by particular groups of
algae (Table 1.1).
In the benthic environment, the size range of
attached algae is even greater – ranging from small
unicells (which colonize freshly exposed surfaces) to
extended filamentous algae of the mature periphyton
1.2 TAXONOMIC VARIATION – THE MAJOR GROUPS OF ALGAE
5
community. Filaments of attached algae such as
Cladophora, for example, can extend several centimetres into the surrounding aquatic medium. These
macroscopic algae frequently have small colonial
algae and unicells attached as epiphytes (Fig. 2.28),
so there is a wide spectrum of sizes within the
localized microenvironment.
Diversity of shape
The shape of algal cells ranges from simple single
non-motile spheres to complex multicellular structures (Fig. 1.2). The simplest structure is a unicellular non-motile sphere (Fig. 1.2b), which may become
elaborated by the acquisition of flagella (Fig. 1.2c),
by a change of body shape (Fig. 1.2a) or by the development of elongate spines and processes (Fig. 1.2d).
Cells may come together in groups without defined
number or shape (Fig. 1.2e) or may form globular
colonies that have a defined morphology (Fig. 1.2f,g).
Cells may also join together to form linear colonies
(filaments) which may be unbranched or branched
(Fig. 1.2h, i).
Although motility is normally associated with the
possession of flagella, some algae (e.g. the diatom
Navicula and the blue-green Oscillatoria) can move
without the aid of flagellae by the secretion of surface mucilage. In many algae, the presence of surface mucilage is also important in increasing overall
cell/colony size and influencing shape.
Size and shape, along with other major phenotypic
characteristics, are clearly important in the classification and identification of algal species. At a functional
and ecological level, size and shape are also important in terms of solute and gas exchange, absorption
of light, rates of growth and cell division, sedimentation in the water column, cell/colony motility and
grazing by zooplankton and (Sigee, 2004).
1.2
Taxonomic variation – the major
groups of algae
Freshwater algae can be grouped into 10 major divisions (phyla) in relation to microscopical appearance
(Table 1.2) and biochemical/cytological characteristics (Table 1.3). Some indication of the ecological and
taxonomic diversity of these groups is given by the
Figure 1.2 General shapes of algae. Non-motile unicells: (a) Selenastrum; (b) Chlorella. Motile unicell: (c)
Chlamydomonas. Non-motile colony: (d) Scenedesmus
(e) Asterionella. Motile colony: (f) Pandorina; (g)
Volvox. Unbranched filament (h) Spirogyra. Branched
filament (i) Cladophora. Reproduced, with permission,
from Bellinger (1992).
number of constituent species (Table 1.2) for freshwater and terrestrial algae in the British Isles (taken
from John et al., 2002), with green algae and diatoms
far outnumbering other groups – reflecting their
widespread occurrence and ability to live in diverse
habitats. Diatoms in particular (over 1600 species)
are ecologically successful, both as planktonic and
benthic organisms. In addition to the above groups,
John et al. (2002) also list other phyla – Raphidophyta
(two species), Haptophyta (five species), Eustigmatophyta (three species), Prasinophyta (13 species) and
Glaucophyta (two species). Although these minor
phyla have taxonomic and phylogenetic interest, they
have less impact in the freshwater environment.
red-brown
various colours
golden brown
golden brown
54
15
115
1652
2
brown
red
yellow-green
73
22
Various colours
grass-green
992
124
blue-green
Typical Colour
297
Index of
Biodiversitya
Microscopic – unicellular
or colonial
Microscopic – unicellular
or filamentous colonies
Microscopic or visible –
unicellular or colonial
Visible – multicellular
cushions and crustose
thalli
Microscopic – unicellular
Microscopic – unicellular
or filamentous
Microscopic – unicellular
Microscopic or visible –
usually colonial
Microscopic or visible –
unicellular or
filamentous colonial
Microscopic – unicellular
Typical Morphology of
Freshwater Species
Data from John et al., 2002.
a Biodiversity: number of species of freshwater and terrestrial algae within the British Isles.
Table adapted from Sigee, 2004.
3. Euglenoids
Euglenophyta
4. Yellow-green algae:
Xanthophyta
5. Dinoflagellates
Dinophyta
6. Cryptomonads
Cryptophyta
7. Chrysophytes
Chrysophyta
8. Diatoms
Bacillariophyta
9. Red algae
Rhodophyta
10. Brown algae
Phaeophyta
1. Blue-green algae
Cyanophyta
2. Green algae
Chlorophyta
Algal Division (phylum)
Table 1.2 Major Divisions of Freshwater Algae: Microscopical Appearance
Non-motile
Gliding movement on
substrate
Non-motile
Some with flagella
Mostly with flagella
Flagellate zoospores and
gametes
All with flagella
Mostly with flagella
Buoyancy regulation Some
can glide
Some unicells and colonies
with flagella
Motility (Vegetative
Cells/Colonies)
Euglena
Colacium
Ophiocytium
Vaucheria
Ceratium
Peridinium
Rhodomonas
Cryptomonas
Mallomonas
Dinobryon
Stephanodiscus
Aulacoseira
Batrachospermum
Bangia
Pleurocladia
Heribaudiella
Synechocystis
Microcystis
Chlamydomonas
Cladophora
Typical Examples
True starchα
Chrysolaminarinβ
Chrysolaminarinβ
Floridean starchα
Laminarinβ
periallo-
β
α, β
α, β, ε
β, ε
α, β
β, ε
a,c2
a,c2
a,c1 ,c2 ,c3
a,c1 ,c2 ,c3
a
a,c1 ,c2 ,c3
fuco-
Pectin, plus
minerals and
silica
Opaline silica
frustule
Walls with
galactose
polymer matrix
Walls with alginate
matrix
Chrysolaminarinβ
α, β
a,c1 ,c2
True starchα
Pectin or pectic
acid wall
Cellulose theca
(or naked)
Cellulose periplast
Paramylonβ
a,b
β, γ
viola-
α, β, γ
a,b
External Covering
Peptidoglycan
matrices or walls
Cellulose walls,
scales
Protein pellicle
β
a
Starch-like
Reserve
Cyano-phycean
starchα
True starchα
zea-
Carotenes
Chlorophylls
Diag.*
Carotenoids
4
2
4
4
4
3
4
3
2
0
Outer
Membranes
3
0
4
3
2
3
3
3
2–6
0
Thylakoid
Groups
Chloroplast
Fine-Structure
2 unequal
(heterokont)
reproductive
cells only
1, reproductive
cells only
0
2 unequal
(heterokont)
2 unequal
(heterokont)
2 unequal
(heterokont)
2 equal (isokont)
0–many. Similar
(isokont)
1–2 emergent
0
Flagella
(Vegetative Cells
& Gametes)
Data from Lee (1997), van den Hoek et al. (1995), John et al. (2002) and Wehr and Sheath (2003).
# Major pigments are shown in bold type.
*Diagnostic carotenoids, used for HPLC analysis (Fig. 2.11): zea- (Zeaxanthin: also present in chlorophytes, cryptophytes), viola- (violaxanthin), peri- (peridinin), allo- (alloxanthin),
fuco- (fucoxanthin, also present in chrysophytes).
Starch-like reserves α:α-1,4 glucan; β:β-1,3 glucan.
Table adapted from Sigee, 2004.
10. Brown algae
Phaeophyta
8. Diatoms
Bacillario-phyta
9. Red algae Rhodophyta
1. Blue-green algae
Cyanophyta
2. Green algae
Chlorophyta
3. Euglenoids
Euglenophyta
4. Yellow-green algae:
Xanthophyta
5. Dinoflagellates
Dinophyta
6. Cryptomonads
Cryptophyta
7. Chrysophytes
Chrysophyta
Algal Division
(phylum)
Pigmentation#
Table 1.3 Major Divisions of Freshwater Algae: Biochemical and Cytological Characteristics
8
1 INTRODUCTION TO FRESHWATER ALGAE
In terms of diversity, freshwater algae also have
a major division into prokaryotes (blue-green algae)
and eukaryotes (remaining groups) based on cell size,
ultrastructure, antibiotic resistance and general physiology. Even within the eukaryote groups, fundamental differences in phenotype and molecular characteristics indicate evolutionary derivation from a range
of ancestral types (polyphyletic origins).
1.2.1
Microscopical appearance
The colour of freshwater algae is an important aspect
their classification (Table 1.2), and ranges from bluegreen (Cyanophyta) to grass green (Chlorophyta),
golden brown (Chrysophyta, Bacillariophyta), brown
(Phaeophyta) and red (Rhodophyta). Variations in
colour are shown in Fig. 1.3, and in the colour
C
G
P
Aph
An
M
100µm
Figure 1.3 Colour characteristics
of different algal groups. Top: Fresh
lake phytoplankton sample showing colour differences between major algal phyla: Dinophyta (brown:
C), Cyanophyta (blue-green: An,
Aph, M) and Chlorophyta (grassgreen: P). Algal genera: An –
Anabaena, Aph – Aphanothece,
C – Ceratium, G – Gomphosphaeria,
M – Microcystis, P – Pandorina.
Bottom left: Synura (cultured alga,
lightly fixed) showing golden brown
colour of Chrysophyta. Bottom right:
End of filament of Aulacoseira. granulata var. angustissima (with terminal spine) from lake phytoplankton showing olive-green chloroplasts
(Bacillariophyta).
20µm
10µm
1.2 TAXONOMIC VARIATION – THE MAJOR GROUPS OF ALGAE
9
photographs of Chapter 4. The use of colour as a taxonomic marker can be deceptive, however, since the
normal balance of pigments may vary. Green algae
living on snow, for example, may have a preponderance of carotenoid pigments – forming a ‘red bloom’
(Hoham and Duval, 2001). Heterotrophic algae take
up complex organic molecules by surface absorption
(organotrophy) or ingestion (phagotrophy), and have
either retained photosynthetic pigments (photoorganotrophs, mixotrophs) or lost pigmentation
completely (obligate heterotrophs) (Sigee, 2004).
Even within a ‘normal’ ecological situation, the
colour of a particular alga can show considerable
variation (see, for example, Anabaena, Fig. 4.24).
Apart from colour, the other obvious characteristics under the light microscope are overall size,
whether the organism is unicellular or colonial and
whether it is motile (actively moving) or non-motile.
Within different groups, algae may be largely unicellular (euglenoids, dinoflagellates, cryptophytes),
multicellular (brown algae) or a mixture of the
two (other groups). Motility (single cells or entire
colonies) is also an important feature, with some algal groups being entirely flagellate (dinoflagellates,
cryptophytes) while others are a mixture of flagellate
and non-flagellate organisms (green algae, xanthophytes). Other groups of algae are entirely without
flagella, but are able to move by buoyancy regulation (blue-greens), gliding movements on substratum
(blue-greens, diatoms) or are entirely non-motile (red
and brown algae).
tion 2.3.3, Fig 2.11), and have been particularly
useful in the analysis of estuarine eutrophication
(Section 3.5.2).
Visualization of key differences in cell structure
normally requires the higher resolution of oil immersion (light microscopy), transmission or scanning
electron microscopy (TEM or SEM) and includes
both internal (e.g. chloroplast fine structure) and external (e.g. location/number of flagella, cell surface
ornamentation) features. Comparisons of light and
electron microscope images are shown in Fig. 1.4
(light/TEM) and Figs. 4.56 and 4.57 (light/SEM).
1.2.2
r Diversity
Biochemistry and cell structure
Major biochemical features of freshwater algae include pigmentation, food reserves and external covering (Table 1.3). Different groups have distinctive combinations of chlorophylls and carotenes,
while only three groups (blue-greens, cryptomonads, red algae) have phycobilins. All pigmented algae have chlorophyll-a, which can therefore be used
for the estimation of total biomass (Chapter 2).
Diagnostic carotenoids have been particularly useful in high-performance liquid chromatography
(HPLC) identification and quantitation of major algal
groups within mixed phytoplankton samples (Sec-
1.2.3
Molecular characterization
and identification
Although identification of algal taxa is normally
based on microscopic characteristics (particularly
morphology and colour), there are a number of situations in which molecular techniques have taken
precedence, such as the following.
r No
clear morphological characteristics are available. This has particularly been the case for unicellular blue-green algae.
r Algae
are relatively inaccessible and difficult to
visualize. This is the case for biofilms, where algae
are enclosed in a gelatinous matrix, and in many
cases are a relatively small component of a very
heterogeneous community of organisms.
is being studied within species, where
strains are often distinguished in biochemical and
genetic terms.
Ultimately, species definition and identification in
both prokaryote and eukaryote algae may depend on
molecular analysis, with determination of unique and
defining DNA sequences followed by development
of species-specific nucleotide probes from these (see
below) .This approach would be particularly relevant
in the case of blue-green algae, but there are a number
of problems in relation to species-specificity in this
group (Castenholz, 1992):
10
1 INTRODUCTION TO FRESHWATER ALGAE
Ch
Figure 1.4 Prokaryote features of
blue-green algae. Top right: phase
contrast (light microscope) image
of live filamentous colony of Anabaena, showing central patches
of pale chromatin (Ch). Top left:
transmission electron micrograph of
whole cell of Anabaena showing central patch of granular chromatin. Bottom: Detail from top
left, showing fine-structural detail.
Ch – central region of chromatin
(no limiting membrane), Ca – carboxysome (polyhedral body), Gy –
glycogen granule (cyanophycean
starch), Th – peripheral thylakoid
membranes, V – vacuole, P – thin
peptidoglycan cell wall.
3µm
1µm
P
Ca
Th
Gy
Ch
V
r Polyploidy
(multiple genomic copies per cell)
may occur, with variation between the multiple
genomes. As many as 10 multiple genome copies
have been observed in some blue-green algae.
r Horizontal
gene transfer means that some DNA
fragments are dispersed over a range of species.
DNA/RNA sequence analysis
Analysis of DNA sequences has been widely used
for the identification of both blue-green (16S rRNA
genes) and eukaryote algae (18S rRNA and chloroplast DNA). This technique has been used by Droppo
et al. (2007), for example, to determine the taxonomic
composition of biofilms – identifying bacteria, bluegreen algae and some eukaryote unicellular algae.
The technique involves:
r collecting
a sample of biomass from the entire
microbial community
r obtaining a DNA sample; this may involve extraction from a mixed environmental sample such as
biofilm or soil (Zhou et al., 1996)
r polymerase chain reaction (PCR) amplification of
a specific nucleotide region – typically 16S or 18S
rRNA genes
11
1.2 TAXONOMIC VARIATION – THE MAJOR GROUPS OF ALGAE
r separation
of the amplified strands by denaturing gradient gel electrophoresis (DGGE), or purification of the PCR products using a rapid purification kit
r sequence analysis with comparison to a standard
database; sequence identification is normally based
on a match of at least 90%.
The pattern of bands in the DGGE gel gives an
estimate of community complexity, and the intensity
of individual bands (derived from individual species)
a measure of population size. In addition to providing
taxonomic information where classical morphological characteristics do not apply, molecular identification also has the advantage that the whole of the
microbial community (communal DNA sample)
is being analysed in an objective way. Nonphotosynthetic bacteria, Archaea, and protozoa
are also identified in addition to prokaryote and
eukaryote algae (Droppo et al., 2007; Galand
et al., 2008).
Potential limitations of molecular analysis are that
identification may be tentative (often only to genus
level) and taxonomic quantitation (relative numbers
of different algae) is difficult. The technique has been
particularly useful in relation to the biodiversity of
marine blue-green unicellular algae, but has also been
used in a number of freshwater systems (Table 1.4)
and at the freshwater/marine interface (nif H genes).
nif H genes Foster et al. (2007) used DNA quantitative PCR (QPCR) technology to amplify and detect
the presence of species-specific nif H genes in bluegreen algae. This gene encodes the iron-containing
protein nitrogenase (the key enzyme involved in
nitrogen fixation), and provides a marker for nitrogen fixing (diazotrophic) algae in the freshwater
environment. The technique was used to demonstrate that the blue-green algal symbiont Richelia
Table 1.4 Molecular Identification of Algal Species in Aquatic Environmental Samples
Environmental Sample
Technique
Reference
Picoplankton in Lake Baikal
Direct sequencing
Colonial blue-greens: Anabaena,
Microcystis, Nodularia
Flagellate nanoplankton
PCR-RFLP analysis of the cpcB-A intergenic
spacer & flanking regions
SSU rRNA probes for Paraphysomonas
(Chrysophyte)
Large subunit rRNA probe for Pseudo-Nitzschia
(Diatom)
Use of ITS-specific PCR assays for Pfiesteria
(Dinoflagellate)
Amplification of 16sRNA genes, with denaturing
gel electrophoresis (DGGE)
Quantitative PCR (QPCR) analysis of the nif H
gene (encodes part of the nitrogenize enzyme)
Sequencing of 18s rRNA genes to identify major
cryptophyte river populations, plus minor lake
populations of diatoms
Population heterogeneity in Spumella
(Chrysophyte) – SSU rRNA sequences
Population heterogeneity in Desmodesmus
(Chlorophyte) – ITS2rRNA sequences
Semenova and
Kuznedelov (1998)
Bolch et al. (1996)
Mixed diatom populations and
laboratory cultures
Estuarine river samples
Laboratory biofilm samples:
blue-greens and unicell eukaryotes
Diazotrophic blue-green algae within
the Amazon river freshwater plume
Arctic ecosystems: Freshwater
stamukhi lake and inflow river
Lake and river flagellate samples
Pond samples
SSU rRNA – small subunit ribosomal RNA.
ITS – internal transcribed spacer.
Caron et al. (1999)
Scholin et al. (1997)
Litaker et al. (2003)
Droppo et al. (2007)
Foster et al. (2007)
Galand et al. (2008)
Pfandl et al. (2009)
Vanormelingen et al.
(2009)
12
1 INTRODUCTION TO FRESHWATER ALGAE
(associated with the diatom Hemiaulus hauckii) was
specifically linked to the Amazon freshwater outflow
(river plume) in the western tropical North Atlantic
(WTNA) ocean, and that the H. hauckii–Richelia
complex could be used as a bio-indicator for pockets
of freshwater within the WTNA ocean.
Molecular probes The development of speciesspecific oligonucleotide probes from DNA sequence
data, followed by in situ hybridization, has considerable potential for the identification and counting of
algae in environmental samples.
As with direct sequencing, this technique has particular advantages with small unicellular algae, where
there are often relatively few morphological features
available for identification. Caron et al. (1999) sequenced the small-subunit ribosomal genes of four
species of the colourless chrysophyte genus Paraphysomonas, leading to the development of oligonucleotide probes for P. imperforata and P. bandaiensis.
Molecular probes have major potential for the detection of nuisance algae, particularly those that produce toxins. They have been used, for example, to distinguish toxic from non-toxic diatom species (Scholin
et al., 1997), where differentiation would otherwise
require the time-consuming application of scanning
and transmission electron microscopy. They have also
been used for the rapid identification of Pfiesteria piscicida, a potentially toxic dinoflagellate that has been
the cause of extensive fish mortalities in coastal rivers
of the eastern United States. Litaker et al. (2003) used
unique sequences in the internal transcribed spacer
(ITS) regions ITS1 and ITS2 to develop PCR assays
capable of detecting Pfiesteria in natural river assemblages. These have been successfully used to detect
the potentially harmful organism in the St Johns River
system, Florida (USA).
1.3
Blue-green algae
Blue-green algae (Cyanophyta) are widely-occurring
throughout freshwater environments, ranging in size
from unicellar forms such as Synechococcus (barely
visible under the light microscope – Figs. 2.16 and
4.31) to large colonial algae such as Microcystis (Fig.
4.34) and Anabaena (Fig. 4.24a). Large colonies of
the latter can be readily seen with the naked eye, and
show a simple globular or filamentous form with
copious mucilage. The balance of photosynthetic
pigments present in blue-green algae (Table 1.3)
varies with light spectrum and intensity, resulting in
a range of colours from brown to blue-green (Fig.
4.24a). The phycobilin pigments are particularly
prominent, with phycocyanin tending to predominate
over phycoerythrin at low light levels, giving the cells
the blue-green colour typical of these algae. It is thus
an advantage to observe material from shaded as well
as better illuminated situations wherever possible.
1.3.1 Cytology
Prokaryote status
The prokaryote nature of these algae is indicated by
the small size of the cells (typically <10µm diameter) and by the presence of central regions of nucleoid
DNA (not enclosed by a nuclear membrane). These
nucleoid regions (Fig. 1.4) can be observed both by
light microscopy (as pale central areas within living
cells) and by transmission electron microscopy (as
granular regions in chemically fixed cells) – where
the absence of a limiting membrane can be clearly
seen. Although these algae lack the cytological complexity of eukaryote organisms (no membrane-bound
organelles such as mitochondria, plastids, microsomes, Golgi bodies) they do have a range of simple granular inclusions such as carboxysomes, cynophycin granules, polyphosphate bodies and glycogen
particles (Fig. 1.4). Photosynthetic pigments are associated with thylakoid membranes, which are typically
dispersed throughout the peripheral protoplasm.
As with other prokaryotes, cell walls are made up
of a peptidoglycan with a lipopolysaccharide layer
outside and are generally quite thin. Some species
also have a mucilage layer outside the cell wall which
may be dense or watery, structured or unstructured.
The outside layers of the cell wall can sometimes
become stained straw coloured or brownish from iron
and other compounds in the surrounding water as in
Scytonema and Gloeocapsa.
The fundamental bacterial nature of these organisms distinguishes them from all other algae, and
determines a whole range of features – including
1.3 BLUE-GREEN ALGAE
molecular biology, physiology, cell size, cell structure and general morphology.
Gas vacuoles
In some species gas vacuoles may be formed, appearing under the light microscope as highly refractive or
quite dark structures. Gas vacuoles aid buoyancy in
planktonic species allowing the cells to control their
position in the water column. Cells may then congregate at a depth of optimal illumination, nutrient
concentration or other factor for that species. This is
not necessarily at the surface as there the light intensity may be too great and cause photoinhibition or
permanent cell damage. Movement up and down in
the water column can enhance nutrient uptake as it allows the cells to migrate to depths where essential nutrients, e.g. phosphates, are more abundant and then
up towards the surface for light energy absorption.
When gas vacuole containing species are present in a
sample of water they often tend to float to the surface
(this can cause problems when preparing the sample
for cell counts; see Chapter 2).
In addition to gas vacuole-mediated movements
of planktonic blue-greens within the water column,
other types of motility also occur – including gliding
movements of filamentous algae (such as Oscillatoria) on solid substrata.
1.3.2
Morphological and taxonomic diversity
Blue-green algae are remarkable within the prokaryote kingdom for showing the range of size and form
noted above, with some organisms forming quite
large, complex, three-dimensional colonies. Limited differentiation (Figs. 4.24a–c) can occur within
colonies, with the formation of heterocysts (nitrogenfixation) and akinetes (thick-walled resistant cells). In
filamentous forms the cells may be the same width
along the filament or in some genera they may narrow
or taper towards the end, even forming a distinct hairlike structure in the case of Gloeotrichia (Fig. 4.22).
Clear branching occurs in the most complex forms,
with a fundamental distinction between ‘true branching’ as in Stigonema (Fig. 4.20), or ‘false branching’
as in Tolypothrix (Fig. 4.21). True branching involves
division of a single cell within the filament, giving
13
rise to two or more daughter cells which themselves
form branches. In contrast, false branching involves
lateral extension of the filament without the single
cell division and daughter cell development as above.
Freshwater blue-green algae can be divided into
four main groups (Table 1.5) in relation to general
morphology, presence/absence of specialized cells
and the nature of branching in filamentous forms.
These four groups form the basis for current taxonomy of this phylum, as adopted by John et al. (2002),
Komarek et al. (2003a,b).
r Chroococcales. The simplest blue-green algae, oc-
curring essentially as solitary cells (no filamentous
forms), typically enclosed by a thin layer of mucilage. The cells may remain as single cells, or
be aggregated into plate-like or globular colonies.
Typically planktonic with some colonial forms (e.g.
species of Microcystis) forming massive surface
blooms containing individual colonies that are recognizable with the naked eye. They typically lack
specialized cells, though one group (typified by
Chaemaesiphon) forms exospores.
r Oscillatoriales. Filamentous algae, lacking heterocysts and akinetes. These relatively simple algae
occur as planktonic (some bloom forming) or benthic aggregations. In some cases they form dense
mats on mud or rocky substrata which secondarily detach as metaphyton into to the main body of
water (Fig. 2.1).
r Nostocales.
A diverse group of filamentous algae, planktonic or benthic, with heterocysts and
akinetes but not showing true branching. Filaments
may be unbranched or show false branching. These
algae are able form large colonies by lateral association of filaments (bundles), 3-D tangles or as
radiating filaments from the centre of the sphere.
r Stigonematales.
Filamentous algae, with heterocysts and akinetes and showing true branching.
Structurally the most complex blue-green algae,
with some thalli (e.g. Fischerella) differentiated
into multiseriate/uniseriate basal filaments and
uniseriate erect branches. Largely benthic algae,
with genera such as Stigonema commonly attached
to substrata in standing and flowing waters, detaching to form planktonic masses.
14
1 INTRODUCTION TO FRESHWATER ALGAE
Table 1.5 Taxonomic and Morphological Diversity in Freshwater Blue-Green Algae
Taxonomic Order
Major Morphotype
Chroococcales
(a) Unicellular to spheroid
colonies, lacking
specialized vegetative,
resistant or reproductive
cells
Colony Size and Form
Attached or Planktonic
Example*
Single cells
Planktonic or attached to plant
and substrate surfaces
Free-floating, tangled-up with
filamentous algae or
attached to substrates
Free-floating or sedentary
Planktonic
Synechococcus (4.31)
Small colonies
(4–32 cells)
Flat plate of cells
Large solid spherical colony
Large hollow spherical colony Planktonic
Chroococcus (4.33)
Gleocapsa (4.32)
Merismopedia
Aphanocapsa (4.35)
Microcystis (4.34)
Gomphosphaeria (4.30)
Coelosphaerium
Chamaesiphon
(b) Unicellular, forming
exospores
Cells remain single or form
multi-layered colonies
Attached to surfaces of
aquatic plants, algae and
inorganic substrate
Oscillatoriales
Filamentous algae, lacking
heterocysts and akinetes
Elongate straight filaments
Planktonic or benthic
Oscillatoria (4.27)†
Phormidium (4.29)
Benthic mat
Planktonic or on mud surfaces Spirulina (4.26)
Elongate spiral filaments
Nostocales
Filamentous algae forming
heterocysts and akinetes,
but no true branching
Stigonematales
Filamentous algae forming
heterocysts and akinetes,
with true branching
Bundles of Elongate filaments Planktonic or benthic
3-D tangle of filaments
Spherical colony of radiating
filaments
Planktonic
Planktonic
Branched mass of filaments
Differentiation into basal
filaments and erect
branches
Benthic or planktonic
Benthic
Aphanizomenon (4.23)
Nostoc (4.25)
Anabaena (4.24)
Gloeotrichia (4.22)
Stigonema (4.20)
Stauromatonema
Fischerella
*Figure numbers in brackets; † Planktonic species of Oscillatoria are placed in the genera Planktothrix, Pseudanabaena or Limnothrix
by some authors.
The range of size and form noted in Table 1.5
indicates a wide morphological diversity, which is
useful in taxonomic identification. The relative importance of molecular versus morphological characteristics in relation to taxonomy reflects the debate
as to whether these organisms should be treated as
bacteria (cyanobacteria – Stanier et al., 1978) or algae (blue-green algae: Lewin, 1976). Although they
fundamentally resemble bacteria in their prokaryote
features, they also differ from bacteria in carrying out
photosynthesis coupled to O2 evolution, and in the
complexity of their morphology. Although current
taxonomy is based primarily on phenotypic charac-
ters (see above), ultrastructural and molecular data
provide useful supplementary information (Komarek
et al., 2003a). Current molecular analyses support
the separation of non-heterocystous and heterocystous genera (Rudi et al., 2000), and are particularly
relevant in the case of unicellular blue-greens, where
morphological features are not adequate. The classical (eukaryote) use of morphology to define species is
also problematic in this group, since the species concept and definition of species is limited by a complete
absence of sexual reproduction.
In the absence of sexual processes, reproduction
in this group is by vegetative or specialized asexual
15
1.3 BLUE-GREEN ALGAE
means. Asexual spores (akinetes) consist of vegetative cells that are larger than normal. They generally have thickened walls and, in filamentous forms,
are often produced next to heterocysts (Fig. 4.24c).
Baeocysts (small spherical cells formed by division
of a mother cell may) be produced in some coccoid species and are released into the environment.
In some filamentous forms deliberate fragmentation
of the filament can occur – releasing the fragments
(hormogonia) to produce new filaments.
1.3.3
50µm
Ecology
Blue green algae are thought to have arisen approximately 3.5 billion years ago (Schopf, 1993) during
which time they have been the dominant form of life
for about 1.5 billion years. As a result of this long evolutionary history, they have adapted to (and frequently
dominate) all types of freshwater environment –
including extreme conditions (thermal springs, desiccating conditions), brackish (semisaline) conditions, high and low nutrient environments and planktonic/benthic habitats. At high latitudes, blue-green
algae are particularly adapted to low temperatures
(Tang & Vincent, 2002) – often dominating the benthic environment by forming dense mats. In milder
climates, these organisms frequently dominate surface waters, where they are able to out-compete
other phytoplankton under eutrophic conditions
(Section 3.2.3).
Algal blooms
In mid to late summer, eutrophic temperate lakes
frequently develop massive populations of colonial
blue-green algae These may rise to the surface of the
lake, forming a thick layer of algal biomass at the
top of the water column, out-competing other algae
and having major impacts on zooplankton and fish
populations (Sigee, 2004). The ability of blue-greens
to out-compete other freshwater algae has been attributed (Shapiro 1990) to a range of characteristics,
including:
r optimum
growth at high temperatures – summer
temperatures
Figure 1.5 Midsummer blue-green algal bloom in a
eutrophic lake. SEM view of lake epilimnion phytoplankton sample (August), showing the dense population of filamentous Anabaena flos-aquae that totally
dominated the algal bloom (chlorophyll-a concentration
140 µg ml−1 ). Copious mucilage associated with this alga
is seen as numerous fine strands, formed during the dehydration preparation process. See also Figs 2.8 (seasonal
cycle) and 4.24 (live Anabaena). Photo: Sigee and Dean.
r low
light tolerance – important within the dense
algal bloom, and enabling algae to survive lower
in the water column
r tolerance of low N/P ratios – allowing continued
growth when N becomes limiting
r depth regulation by buoyancy – avoiding photoinhibition during the early phase of population increase, and allowing algae to obtain inorganic nutrients from the hypolimnion when the epilimnion
becomes depleted in mid to late summer
r resistance
to zooplankton grazing – limiting the
impact of herbivory on algal growth
r tolerance of high pH/low CO2 concentrations – allowing continued growth of blue-greens (but not
other algae) at the lake surface during intense
bloom formation
r symbiotic association with aerobic bacteria – bacterial symbionts at the heterocyst surface maintain
16
1 INTRODUCTION TO FRESHWATER ALGAE
Table 1.6 Ecological Diversity of Freshwater Blue-Green Algae
Major Ecosystem
Specific Conditions
Benthic Algae
Planktonic Algae
Standing waters – lakes and
ponds
Nutrient status: meso- to
eutrophic
Gloeotrichia: Attached to
substrates. Detached
colonies planktonic
Colonial blue-greens:
Microcystis, Anabaena
Oligotrophic
Hard and soft waters
Soft, acid lakes
Wetlands
Soft waters – common in
bogs
Open water in bogs
Running waters – streams
and rivers
Extreme environments
Polar lakes, ponds, streams
Thermal springs
Brackish waters –
saline lakes
Low temperature, low
light (ice cover)
45 ◦ C or greater
range of salinity
the local reducing atmosphere required for nitrogen
fixation and are also an important source of inorganic nutrients in surface-depleted waters (Sigee,
2004).
Species preferences
Many blue-green algae can tolerate a wide range of
environmental conditions, so strict ecological preferences are unusual in this group. Microcystis aeruginosa (Fig. 4.34) for example can form massive
growths in eutrophic lakes, but low numbers may
also occur in oligotrophic waters (Reynolds, 1990).
The distinction between planktonic and benthic organisms is also frequently not clear-cut. Microcystis is regarded as a typical planktonic alga, but may
also occur as granular masses on lake bottoms, and
(as with the majority of temperate lake algae) overwinters on lake sediments rather than in the water
Unicellular blue-greens:
Synechococcus
Aphanothece
Tolypothrix: Planktonic or
in submerged vegetation
Cylindrospermum: often
forming dark patches on
submerged vegetation
Chroococcus – Attached to substrates, mixed with tangles
of filamentous algae or free-floating
Merismopedia – typical of bog-water communities
Aphanocapsa
Benthic mats:
Oscillatoria rubescens
Oscillatoria tenuis
Benthic mats: Oscillatoria
(Tang et al. 1997)
Mastigocladus laminosus
Oscillatoria (pigmented)
(Darley, 1982)
Synechococcus lividus
(Stal, 1995)
Aphanothece halophytica
(Yopp et al., 1978)
column. Many benthic algae become detached, and
either remain loose on the substratum or rise in the
water column and become free-floating. Colonies of
Gloeotrichia, for example, often detach from substrates and become planktonic (Fig. 4.22), where they
grow and may reach bloom proportions.
In spite of these cautions, many blue-green
algae can be characterized in relation to particular
environments and whether they are typically attached
or planktonic. Some examples are shown in Table
1.6, for both ‘normal’ and ‘extreme’ environments.
As with other algae, ecological preferences typically
relate to multiple (rather than just single) environmental factors. Colonial blue-green algae, which
grow particularly well in high-nutrient lakes are
also suited to hard waters (high Mg, Ca) and to the
alkaline conditions typical of these environments.
Conversely, oligotrophic lakes, which support
unicellular rather than colonial blue greens, tend to
be soft, slightly acid waters.
17
1.4 GREEN ALGAE
1.3.4
Blue-green algae as bio-indicators
As with other algal groups, the presence or absence
of particular species can be a useful indicator of
ecological status (Table 1.6). The dominant presence
of colonial blue-greens, forming dense summer
blooms (Fig. 1.5) has been particularly useful as
an indicator of high nutrient status, and these algae
are a key component of various trophic indices
(Chapter 3). Conversely, populations of unicellular
blue-green algae are indicative of oligotrophic to
mesotrophic conditions.
1.4 Green algae
In the freshwater environment, green algae (Chlorophyta) range in size from microscopic unicellular organisms to large globular colonies and extensive filamentous growths. They are characterized by a fresh
green coloration due to the presence of chlorophylls-a
and -b, which are not obscured by accessory pigments
such as β-carotene and other carotenoids. In exceptional cases the carotenoid pigments may occur in
large amounts, obscuring the chlorophylls and giving
the alga a bright red colour. This is seen with the flagellate Haematococcus (Fig. 4.54), which frequently
colours bird baths and other small pools bright red,
and the snow alga Chlamydomonas nivalis – where
one of the red carotenoids has a major photoprotective
function.
1.4.1
Cytology
In addition to their characteristic pigmentation and
other biochemical features (Table 1.3), green algae
also have a number of distinctive cytological aspects.
r Flagella, when present, occur in pairs. These are
equal in length without tripartite tubular hairs, and
have a similar structure (isokont). Some genera
have four or even eight flagella, but this is unusual.
r Chloroplasts
vary in shape, size and number. In
unicellular species they tend to be cup-shaped
(Fig. 4.55), but in filamentous forms may be an-
nular (Fig. 4.19), reticulate (Fig. 4.17), discoid or
ribbon-like (Fig. 4.13). They have a double outer
membrane, with no enclosing periplastidal endoplasmic reticulum.
r Production
and storage of the photosynthetic reserve (starch) occurs inside the plastid, with granules frequently clustered around the pyrenoid. In all
other eukaryote algae the storage material occurs
mainly in the cytoplasm.
In motile species an eyespot is frequently present,
appearing red or orange (Fig. 4.38) in fresh specimens. The cell wall in green algae is made of
cellulose.
1.4.2
Morphological diversity
Green algae are the most diverse group of algae, with
about 17 000 known species (Graham and Wilcox,
2000). This diversity is reflected by the varied of morphology, with organisms being grouped into a range
of growth forms – depending on whether they are
unicellular, colonial or filamentous (Table 1.7).
The level of greatest morphological and reproductive
complexity is represented by Charalean algae (e.g.
Chara, Nitella), which can reach lengths of over a metre, have whorls of branches at nodes along the length
of the thallus and have been frequently confused with
aquatic higher plants such as Ceratophyllum.
In the past, this morphological diversity provided
the taxonomic basis for green algal classification
(Bold and Wyne, 1985) – with orders primarily being
defined largely on a structural basis. These included
the orders Volvocales (flagellate unicells and simple
colonial forms), Chlorococcales (unicells and nonmotile coenobial colonies), Ulotrichales (unbranched
filaments) and Chaetophorales (branched filaments).
More recently, the application of cytological, comparative biochemical and molecular sequencing techniques has demonstrated the occurrence of extensive
parallel evolution. On the basis that classification
should reflect phylogeny, these original groupings
are thus no longer valid, and a new classification is
emerging where individual orders contain a mixture
18
1 INTRODUCTION TO FRESHWATER ALGAE
Table 1.7 Range of Morphologies in Freshwater Green Algae
Major Morphotype
Attached or Planktonic
Example*
Flagellate unicells
Nonflagellate Unicells
Planktonic
Planktonic or benthic (often
associated with periphyton)
Colonial
Planktonic
Disk-shaped plate, one
cell thick
Unbranched filaments
Attached to higher plants
(epiphyte) or to stones
Typically attached, but may
detach to become planktonic
Chlamydomonas (4.55)
Crescent shaped – Selenastrum (4.78)
Equatorial division – Micrasterias (4.81);
Closterium (4.80)
Net – Hydrodictyon (4.1)
Hollow sphere – Volvox (4.40), Coelastrum,
Eudorina (4.39)
Solid sphere – Pandorina (4.36)
Plate of cells – Gonium (4.38), Pediastrum (4.47)
Small linear colonies – Scenedesmus (4.50)
Branching colonies – Dictyosphaerium (4.45)
Coleochaete, Chaetosphaeridium
Branched filaments
Large complex algae with
whorls of branches
Attached
Uronema, Microspora, Oedogonium (4.17),
Zygnema (4.15), Spirogyra (4.13)
Chaetophora, Draparnaldia, Cladophora (4.6)
Chara, Nitella
*Figure numbers in brackets.
unicells, globular colonial and filamentous forms
(Graham and Wilcox, 2000).
1.4.3
Ecology
Green algae are ecologically important as major producers of biomass in freshwater systems, either as
planktonic (standing waters) or attached (running waters) organisms – where they respectively may form
dense blooms and periphyton growths.
Algal blooms
In mesotrophic and eutrophic lakes, green algae do
not normally produce the dense blooms seen with
diatoms and blue-green algae – but do become dominant or co-dominant in early summer, between the
clearwater phase and midsummer mixed algal bloom.
In smaller ponds, filamentous Spirogyra and colonial Hydrodictyon frequently form surface blooms or
scums, while in garden pools and birdbaths Haematococcus may form dense populations. When nutrient
levels become very high, a switch may occur from
colonial blue-green to green algae as major bloom
formers. This is can be seen in some managed fishponds, where organic and inorganic nutrients are applied to enhance fish production by increasing carbon flow through the whole food chain. In the Trebon
basin biosphere reserve (Czech Republic), application of lime (supply of carbonate and bicarbonate
ions), organic fertilizers and manure leads to high pH
and hypertrophic nutrient levels. In these conditions,
a short diatom bloom (Stephanodiscus) is replaced in
early summer by populations of rapidly growing unicellular and small colonial green algae (Scenedesmus
and Pediastrum) which continue to dominate for
the rest of the annual cycle (Pokorny et al., 2002a;
Pechar et al., 2002). Extensive growth of attached
algae (periphyton) may also occur under high nutrient conditions. Branched thalli of Cladophora can
form extended communities of shallow water vegetation along the shores of eutrophic lakes and streams,
breaking off in storms to form mass of loose biomass
which can degrade to generate noxious odours.
Another filamentous alga, Mougeotia, can form large
mucilaginous subsurface growths in lakes that have
been affected by acid rain.
19
1.4 GREEN ALGAE
Table 1.8 Ecological Diversity of Freshwater Green Algae
Major Ecosystem
Specific Conditions
Benthic Algae
Planktonic Algae
Standing waters – lakes and
ponds
Wide range of conditions
Oedogonium
Eutrophic to hypertrophic
Oligotrophic
High (H) and low (L) pH
Cladophora
Scenedesmus, Pediastrum.
Tetraedron
Chlamydomonas
Scenedesmus, Pediastrum.
Desmids
Mougeotia (L)
Standing waters – wetlands
Running waters – streams
and rivers
Running waters –
Estuaries, brackish water
Specialized
microenvironments
Hard water – high (Ca)
concentration
Salt lakes
Wide range of conditions
Chara, Nitella (H)
Coleochaete (L)
Hydrodictyon
Small unicells, Dunaliella
Selenastrum
Chlamydomonas
Spirogyra, Mougeotia, desmids
Chlamydomonas, Spirogyra
Low nutrient, low pH bogs
Wide range of conditions
Hardness (Ca
concentration)
Wide range of conditions
High salinity
Very high nutrients, e.g.
High sewage level
Endosymbiont in
invertebrates
Associated with Calcareous
deposits
Hydrodictyon
Prototheca
Scenedesmus
Dunaliella
Euglena
Chlorella*
Oocardium stratum
*Also widely occurring as a free-living organism.
Species preferences
Individual species of green algae differ considerably
in their ecological preferences, ranging from broad
spectrum organisms (Spirogyra, Chlamydomonas) to
species with very restricted habitats.
Spirogyra occurs in a wide range of habitats, where
it is typically attached to stable substratum (as periphyton) but also occurs as free floating mats (Lembi
et al., 1988) – derived either by detachment of from
periphyton (vegetative propagation) or from benthic zygotes (sexual derivation). In surveys of North
American sites, McCourt et al. (1986) recorded Spirogyra at nearly a third of all locations, and Sheath
and Cole (1992) detected Spirogyra in streams from
a wide range of biomes – including desert chaparral,
temperate and tropical rainforests and tundra.
Other green algae show clear preferences for particular environmental conditions (Table 1.8) that include degree of water movement (lentic versus lotic
conditions), inorganic nutrient status (oligotrophic
to eutrophic), pH, hardness (Ca concentration) and
salinity. These conditions frequently occur in combination, with many moorland and mountain water bodies being low nutrient and neutral to acidic,
while lowland sites in agricultural areas are typically
eutrophic and alkaline.
Nutrient status Many desmids (e.g. Closterium)
are particularly characteristic of oligotrophic (low nutrient) lakes and ponds – often in conditions that
are also slightly acidic (see above) and dystrophic
(coloured water). Desmid diversity is particularly
high in these conditions, sometimes with hundreds
20
1 INTRODUCTION TO FRESHWATER ALGAE
of species occurring together at the same site
(Woelkerling, 1976). Desmids are also typical of
nutrient-poor streams, where they are permanent residents of periphyton – making up to 10% of total
community biomass. These desmids are associated
with plants such as the bryophyte Fontinalis, achieving concentrations as high as 106 cells g−1 of substrate (Burkholder and Sheath, 1984). Desmids often
constitute a significant proportion of algal biomass in
wetlands (bogs and fens) where they are also a major
aspect of species diversity.
Some species of desmids – such as Closterium
aciculare, are more typical of high nutrient, slightly
alkaline lakes – and are indicators of eutrophic conditions. Periphytic algae typical of eutrophic waters
include Cladophora.
Acid lakes Although desmids are typical of neutral to slightly acid lakes, other green algae are
adapted to more extreme conditions. These include
the filamentous green alga, Mougeotia, which can
form substantial sub-surface growths in acidified waters – and is widely regarded as an indicator of early
environmental change (Turner et al., 1991). Whether
acidification is the result of acid precipitation, experimentation (Webster et al., 1992) or industrial pollution, the acid conditions also tend to be associated
with:
r relatively clear waters, due to low levels of phytoplankton
r increases in the concentration of metals such as Al
and Zn
r reduced
levels of dissolved organic carbon,
with derivation largely from external lake (allochthonous) rather than internal (autochthonous)
sources
r food
web changes, including a reduction in the
numbers of herbivores.
Laboratory studies (Graham et al., 1996a,b)
showed that Mougeotia was physiologically adapted
to such conditions, with the ability to photosynthesis over a wide range of irradiances (300–2300 µmol
quanta m−2 s−1 ) and tolerance to a broad range of
both pH (pH 3–9) conditions and metal concentrations. The physiological adaptations of this alga, coupled with reduced grazing from herbivores, probably
accounts for the extensive growth and domination of
the benthic environment in many acidic waters.
Alkaline and calcium-rich waters Some
planktonic (Closterium aciculare) and benthic algae
(e.g. Nitella and Chara) are adapted to grow in alkaline habitats, many of which are also rich in calcium
(hard waters). Nitella produces lush meadows on the
bottoms of neutral to high pH lakes, and Chara can
form dense, lime encrusted lawns in shallow alkaline
waters. Hydrodictyon is also typical of hard-water
conditions, occasionally blanketing the surface of
ponds and small lakes, and occurring widely in larger
alkaline standing waters and hardwater streams. Extensive growths of this alga can also be observed in
agricultural situations such as irrigation ditches, rice
paddy fields and fish farms – where there is some
degree of eutrophication.
Some calcium-loving algae occur in very restricted
environments. The unusual, slow-growing desmid
Oöcardium stratum, for example, occurs in calcareous streams and waterfalls at the top of branched calcareous tubes, in association with deposits of ‘travertine’ and ‘tufa’.
Saline waters Various green algae are adapted to
saline conditions in salt lakes and estuaries, where
salt levels range from low (brackish waters) to high
levels. With some widely occurring genera such as
Scenedesmus, brackish conditions are at one end of
the continuum of environmental tolerance. Other algae, such as Dunaliella, are specifically adapted to
a more extreme situation. This organism occurs in
the highly saline waters of commercial salt pans
(salterns) and natural salt lakes such as the Great Salt
Lake of Utah (USA). Dunaliella is able to tolerate external salt concentrations of >5 M, by balancing the
high external osmotic pressure (OP) with a high internal OP generated by photosynthetically produced
glycerol.
Specialized environments In addition to the
free-living occurrence of particular algae in lakes and
rivers that have a range of characteristics (see above),
other green algae are adapted to more specialized
21
1.5 EUGLENOIDS
conditions. Some of these involve a change in the
mode of nutrition from autotrophic to heterotrophic,
and include various species of the non-flagellate
unicell Chlorella – a widespread endosymbiont of
freshwater invertebrates. Other green algae retain
their free-living existence, but are able to supplement
photosynthesis (mixotrophy) by absorption of
exogenous dissolved carbon (Tuchman, 1996) such
as amino acids and sugars through their cell surface
(osmotrophy). The ability to use external organic
carbon may be important in two situations – when
photosynthesis is limiting, and when external soluble
organic material is in excess.
r Limiting photosynthesis. This may occur, for example, under conditions of chronic low light intensity (e.g. arctic lakes). It is also typical of many
acid lakes, where low pH reduces the level of dissolved inorganic carbon to levels that are unable
to saturate photosynthesis. Laboratory experiments
on Coleochaete, an acidophilic alga have demonstrated an ability to use dissolved organic carbon
in the form of hexose sugars and sucrose (Graham
et al., 1994)
aquatic environment. Filamentous green algae, for
example, often dominate environments stressed by
cultural eutrophication, acidification and metal contamination (Cattaneo et al., 1995).
Fossil algae
Green algae do not typically produce resistant
walls that persist in aquatic sediments, and are
much less useful than diatoms and chrysophytes as
bioindicators in terms of the fossil record. There are
some exceptions to this, where the cell wall contains
fatty acid polymers known as algaenans, that can
withstand millions of years of burial (Gelin et al.,
1997). Scenedesmus and Pediastrum are among the
most common members of green algae that have
fossil records, containing algaenans and also some
silicon. The desmid Staurastrum is also frequently
seen as fossil remains, due to the impregnation of
cell wall material with polyphenolic compounds
which confer resistance to bacterial decay (Gunnison
and Alexander, 1975).
r High levels of external dissolved organic carbon.
Some osmotrophic green algae are photoheterotrophic, only able to use organic carbon when
light is present, and when their photosynthesis
is inhibited by the high availability of dissolved
organic carbon (Lewitus and Kana, 1994). The
colourless unicellular alga Prototheca has completely lost the ability to photosynthesize and obtains all its carbon from external sources present
in soils and freshwater environments that are contaminated by sewage. This evolutionary relationship of this obligate heterotroph to green algae
is indicated by the presence of starch-containing
plastids.
1.4.4
Green algae as bioindicators
Contemporary algae
Habitat preferences of contemporary green algae
(Table 1.8) are frequently useful in providing information on physicochemical characteristics of the
1.5
Euglenoids
Euglenoid algae (Euglenophyta) are almost entirely
unicellular organisms, with a total of 40 genera worldwide (about 900 species) – most of which are freshwater. Cells are typically motile, either via flagella or
(in non-flagellate cells) by the ability of the body to
change shape (referred to as ‘metaboly’).
About one-third of the Euglenoids are photosynthetic and classed amongst the algae. The rest are
colourless, being either heterotrophic or phagotrophic, and are usually placed in the Protozoa. In
photosynthetic organisms, pigmentation is closely
similar to the green algae (Table 1.3), but the variable
presence of carotenoid pigments means that these
organisms can routinely vary in colour from fresh
green (Fig. 4.51) to yellow-brown. In some situations, the accumulation of the carotenoid astaxanthin gives cells a bright red coloration. This is seen
particularly well in organisms such as Euglena sanguinea, which forms localized blooms in ponds and
ditches.
22
1.5.1
1 INTRODUCTION TO FRESHWATER ALGAE
Cytology
Euglenoids are typically elongate, spindle shaped
organisms (Figs 1.6 and 4.51) and usually contain
several chloroplasts per cell, which vary in appearance from discoid to star, plate or ribbon-shaped.
Distinguishing cytological features of euglenoids
(Table 1.3) include the following.
r The presence of an anterior flask shaped depression
or reservoir within which the flagella are inserted.
Although two flagella are present only one emerges
from the reservoir into the surrounding medium,
the second being reduced and contained entirely
within the reservoir. An eyespot is often present
and is located close to the reservoir.
r The
production of paramylon as storage reserve.
This β-1,3-linked glucan does not stain blue-black
with iodine solution and is found in both green and
colourless forms.
r The presence of a surface coat or pellicle, which
gives the cell a striated appearance. This occurs just
below the plasmalemma and is composed of interlocking protein strips that wind helically around the
cell. In some the pellicle is flexible (allowing the
cell to change its shape), while in others the pellicle
is completely rigid giving a permanent outline to
the cell. Genera such as Trachelomonas have cells
surrounded by a lorica the anterior end of which is
a narrow, flask-shaped opening through which the
flagellum emerges.
Reproduction is asexual by longitudinal division,
with sexual reproduction being completely unknown
in this group of organisms.
1.5.2 Morphological diversity
Figure 1.6 Diagrammatic view of Euglena, showing
major cytological features. Reproduced, with permission, from Graham and Wilcox (2000). See also Fig.
4.51 (live Euglena).
Almost all euglenoids are unicellular, with colonial
morphology being restricted to just a few organisms where cells are interconnected by mucilaginous strands. One of these is Colacium, a stalked
euglenoid that is widespread on aquatic substrates –
but is particularly common attached to zooplankton
such as Daphnia. In the sessile state, this organism
does not have emergent flagella on the cells – which
occur at the end of the branched mucilaginous attachment stalks. Individual cells may produce flagella, however, swim away and form a new colony
elsewhere.
With the relative absence of colonial form, diversity in this group is based mainly on variation in ultrastructural features – including feeding apparatus,
flagella and pellicle structure (Simpson, 1997). A further aspect of diversity is the distinction between
green and colourless (autotrophic/heterotrophic)
cells, with approximately two thirds of all species
being heterotrophic. Some euglenoids (e.g. strains
and species of Euglena) are facultative heterotrophs,
able to carry out heterotrophic nutrition when photosynthesis is limiting or when surrounding concentrations of soluble organic materials are high. In
the heterotrophic state, these organisms retain their
plastids as colourless organelles and absorb soluble
1.6 YELLOW-GREEN ALGAE
nutrients over their whole surface (osmotrophy).
Other euglenoids (Petalomonas, Astasia, Peranema)
are obligate heterotrophs and have lost their plastids
completely. Many of these organisms consume particulate organic material (phagocytic) and have evolved
a complex feeding apparatus.
1.5.3
Ecology
Euglenoids are generally found in environments
where there is an abundance of decaying organic
material. This is in line with the heterotrophic nature of many of these organisms and the ability to
take up complex organic material either in the soluble or particulate state. Typical habitats include
shallow lakes, farm ponds, wetlands, brackish sand
and mudflats. Within these environments, euglenoids
are particularly associated with interfaces such as
sediment-water and air-water boundaries (Walne
and Kivic, 1990) and should probably not be regarded as open water truly planktonic algae (Lackey,
1968).
Certain euglenoid algae are able to tolerate
extreme environmental conditions. One of these,
Euglena mutabilis, is able to grow in very low pH
waters. This alga has a pH optimum of pH 3.0, can
tolerate values below pH 1.0, and is typical of acidic
metal-contaminated ponds and streams draining
mines. Other euglenoids, found in brackish habitats,
are able to tolerate wide ranges in salinity (Walne
and Kivic, 1990).
1.5.4
Euglenoids as bioindicators
Euglenoid algae are not particularly useful as environmental bioindicators, either in terms of contemporary populations or fossil records. Although
present-day algae show some adaptations to specific
environments (see above), there is no established environmental library and species may be difficult to
identify. Within lake sediments, the lack of calcified or silicified structures that are resistant to decay
means that hardly any remains have survived in the
fossil record.
23
1.6 Yellow-green algae
Yellow-green algae (Xanthophyta) are non-motile,
single celled or colonial algae, with a distinctive pigmentation that gives the cells a yellow or fresh green
appearance (e.g. Fig. 4.18 – Tribonema). Although
there is a wide range in morphology (see below), this
phylum contains relatively few species (compared to
major groups such as green algae) and the algae tend
to be ecologically restricted to small water bodies and
damp soils.
1.6.1
Cytology
Yellow-green algae are most likely to be confused
with green algae when observed in the fresh condition, but differ from them (Table 1.3) in a number of
key features.
r Distinctive pigmentation, with chlorophylls (a, c1
and c2 – but not b), carotenoids (especially βcarotene) and three xanthophylls (diatoxanthin,
vaucheriaxanthin, heteroxanthin).
r Carbohydrate storage as oil droplets or chrysolaminarin (usually referred to as leucosin) granules.
r Walls
composed mainly of pectin or pectic acid
(sometimes with associated cellulose or siliceous
material). The walls often occur as two spliced
and overlapping sections, breaking into ‘H’-shaped
pieces on dissociation of the filaments.
r When flagella are present they are of unequal length
(heterokont) the longer one pointing forward and
the shorter (which can be more difficult to see using
a light microscope) sometimes pointing forwards or
backwards
r Chloroplast fine structure – four outer membranes
(incorporating an endoplasmic reticulum) and thylakoids occurring in threes.
Cells contain two or more discoid and green to
yellow-green chloroplasts, with associated pyrenoids
rarely seen. An eyespot may be present.
24
1.6.2
1 INTRODUCTION TO FRESHWATER ALGAE
Morphological diversity
Yellow-green algae show a range of morphology,
from unicellular to colonial and filamentous (Table 1.9). Unicellular forms are non-flagellate in the
mature (vegetative) state, with motility being restricted to biflagellate zoospores (asexual) or motile
gametes (sexual) reproductive stages. The zoospores
have unequal and morphologically dissimilar flagella (heterokont algae – Fig. 1.7). Unicellular algae include simple free-floating forms (Botrydiopsis, Tetraedriella), or may have associated rhizoidal
systems or an attachment stalk. Some yellow-green
algae form simple globular colonies, which may be
mucilaginous and free floating (e.g. Gloeobotrys)
or attached via a stalk (Ophiocytium). Filamentous
forms may be unbranched (Tribonema – Fig. 4.18)
or branched (Heterococcus), and reach their most
massive state in the coenocytic siphonaceous genera
Botrydium and Vaucheria (Fig. 4.3).
1.6.3
Ecology
Yellow-green algae are rather limited in their exploitation of aquatic habitats, tending to occur on
damp mud (at the edge of ponds) and soil, but not
occurring extensively in either lentic or lotic systems. Where planktonic forms do occur, they tend
to be present in ditches or small ponds. Tribonema
species are often found as free-floating filaments in
temporary waters, and Botrydiopsis arrhiza is found
at the edge of ponds and in patches of water in
sphagnum bogs, where it may form yellowish water blooms. Even Gloeobotrys limneticus, which has
widespread occurrence as mucilaginous colonies in
lake plankton, never forms extensive or dominant
populations (as seen, for example, by blue-green algae). Various yellow-green algae are epiphytic, including Mischococcus and Ophiocytium (attached to
filamentous algae) and Chlorosaccus (attached to
macrophytes).
As with other algal groups, some yellow-green algae have developed alternative modes of nutrition.
Chlamydomyxa is an amoeboid, naked form that
has retained its photosynthetic capability (still has
chloroplasts) but has also become holozoic, ingesting desmids, diatoms and other algae and digesting
them within internal food vacuoles. Chlamydomyxa
is typical of low-nutrient acid bogs, and the holozoic
mode of nutrition may be important for supplementing nitrogen uptake in such adverse conditions.
1.6.4
Yellow-green algae as bioindicators
Yellow-green algae have not been widely used
as bioindicator organisms, partly because they are
Table 1.9 Range of Morphologies in Yellow-Green Algae
Major Morphotype
Attached or Planktonic
Example*
Unicells
Coccoid
Pear-shaped, with rhizoidal system
Ovoid cells on stalk
Amoeboid
Planktonic or benthic
Present on mud surface
Epiphytic on attached or planktonic algae
Benthic, often in hollow cells of Sphagnum
Botrydiopsis
Botrydium
Characiopsis
Chlamydomyxa
Colonial
Small to large globular colonies
Branched colony on stalk
Planktonic
Planktonic or attached
Gloeobotrys
Ophiocytium
Typically planktonic, occasionally attached
when young
Attached, damp soils
Attached
Tribonema (4.18)
Filaments
Unbranched
Branched
Coenocytic, (siphonaceous)
*Figure numbers in brackets
Heterococcus
Botrydium, Vaucheria (4.3)
1.7 DINOFLAGELLATES
25
1.7 Dinoflagellates
Dinoflagellates (Dinophyta) are mostly biflagellate
unicellular algae, although some (e.g. Stylodinium)
are without flagella and are attached. They are predominantly found in the surface waters of marine
systems (about 90% are marine), with only about
220 species present in freshwater environments. Dinoflagellates contain chlorophylls-a and -c, but are
typically golden- or olive-brown (Fig. 1.3) due to the
major presence of carotene and the accessory xanthophyll peridinin. The chloroplasts can be plate-like
(Fig. 4.57) in some species or elongate in others.
Pyrenoids are present, and the main storage product
is starch. Lipid droplet reserves may also be found,
and some dinoflagellates possess an eyespot.
1.7.1
Cytology
Dinoflagellates have a number of distinctive cytological features.
r A large central nucleus (usually visible under the
Figure 1.7 Line drawing of heterokont zoospore, typical of xanthophyte algae, with a long anterior flagellum
bearing two rows of stiff hairs plus a short posterior flagellum. This is smooth and oftemn bears a swelling (FS)
that is part of the light-sensing system. ES – eyespot, G
– Golgi body, M – mitochdondrion, N – nucleus, P –
plastid. Reproduced, with permission, from Graham and
Wilcox (2000).
not a prominent group in the aquatic environment.
Different species do have clear environmental preferences, however, that could be used to provide information on ambient conditions. These are discussed by
John et al. (2002), and include algae that are prevalent
in acid bogs (Botrydiopsis spp., Centritractus spp.),
calcareous waters (Mischococcus spp., Ophiocytium
spp.), humic waters (Botrydiopsis spp., Tribonema
minus), organically-rich conditions (Chlorosaccus
spp.), inorganic nutrient-enriched waters (Goniochloris fallax) and brackish (partly marine) environments
(Vaucheria prolifera, Tetraedriella spp.).
light microscope) containing chromosomes that are
typically condensed throughout the entire cell cycle. These have an unusual structure, with genetically active (transcriptional) DNA on the outside
and genetically inactive DNA (structural DNA)
forming a central core (Sigee 1984).
r Cell wall material lies beneath the cell membrane,
in contrast to many other algae – where the cell
wall, scales or extracellular matrix occur on the
outside of the plasmalemma. The cell wall in
dinoflagellates is composed of cellulose (within
subsurface membrane-bound vesicles), forming
discrete discs (thecal plates), which give the
dinoflagellate a distinctive armoured appearance.
The presence of thecal plates is difficult to see
in light microscope images of living cells, but is
clear when chemically-fixed cells are viewed by
scanning electron microscopy (Fig. 4.57). The
number, shape and arrangement of thecal plates is
taxonomically diagnostic (compare Figs 1.8 and
1.9). A typical arrangement of plates can be seen
in Peridinium (Fig. 1.8), with a division into two
26
1 INTRODUCTION TO FRESHWATER ALGAE
Epitheca
Cingulum
Transverse
Flagellum
Hypotheca
B
Longitudinal Flagellum
Sulcus
A
3'
2'
4'
7''
5'''
C
3'
1'
1''
3''
1'''
2'''
1''''
3a
2a
4''
1''''
D
3'''
5''
4'''
2''''
Figure 1.8 Dinoflagellate symmetry and plates. Top: Diagrammatic view of typical dinoflagellate cell showing flagellar
insertion and grooves within the plate system. A – ventral view; B – dorsal view. Bottom: Pattern of numbered thecal
plates characteristic of the dinoflagellate Peridinium. C – Ventral view; D – dorsal view. See also Fig. 4.57. Reproduced,
with permission, from Wehr and Sheath (2003).
major groupings, forming an apical epitheca (or
epicone) and a posterior hypotheca (or hypocone).
Heavily armoured species tend to be more angular
in outline, and plates may be extended in some
species into elaborate projections. The presence of
such projections or horns in Ceratium, for example
(Fig. 1.9), increases the surface area to volume
ratio of the cell and possibly reduces its sinking
rate by increasing frictional resistance. Projections
may also help in reducing predation. In contrast to
armoured dinoflagellates, some species have very
thin plates or they may be absent altogether (naked
dinoflagellates).
1.7.2 Morphological diversity
These biflagellate unicellular organisms are of two
main types, depending on the point of insertion of the
flagella.
r Desmokont
dinoflagellates, where the two flagella emerge at the cell apex (e.g. Prorocentrum).
Relatively few species occur in this group, which
is also characterized by the presence of two
large plates (valves) covering a major portion of
the cell.
1.7 DINOFLAGELLATES
27
ventral region of the cell. Beating of the flagella –
one of which is extended while the other is contained in the equatorial groove – gives the cell a
distinctive rotatory swimming motion. The term
‘dinoflagellate’ is derived from the Greek word
dineo, which means ‘to whirl.’
Dinoflagellates do not show the range of morphology, from unicellular to colonial forms, seen in other
algal groups. There are some unusual non-flagellate
amoeboid, coccoid and filamentous forms, however,
which reveal their phylogenetic relationship to mainstream dinoflagellates by the characteristic biflagellate structure of their reproductive cells - known as
dinospores (zoospores).
1.7.3
25µm
Figure 1.9 Morphology of Ceratium. Top: Diagrammatic views of mature cell (left) and cyst formation
(right). Bottom: Central region of mature cell (SEM
preparation) showing details of equatorial groove and
thecal plates with pores. See also Figs 2.7 (cyst) and 4.56
(whole cells).
r Dinokont cells, where the flagella emerge from the
middle of the cell (e.g. Ceratium, Peridinium). Dinokont cells have a distinct symmetry (Fig. 1.8)
with dorsoventral division into epitheca and hypotheca (see above). These are separated by an
equatorial groove (cingulum), with a further small
groove – the sulcus, extending posteriorly within
the hypocone. The two are flagella inserted in the
Ecology
In freshwater environments, dinoflagellates are typically large-celled organisms such as Ceratium, Peridinium and Peridiniopsis. Their large size reflects the
high nuclear DNA levels (large amounts of genetically inactive chromatin) and correlates with a long
cell cycle and low rate of cell division. These features
are typical of organisms that are K-strategists, dominating environments that contain high populations of
organisms living under intense competition (Sigee,
2004).
Dinoflagellates are meroplanktonic algae, present
in the surface waters of lakes and ponds at certain
times of year. The annual cycle is characterized by
two main phases.
r A midsummer to autumn bloom, when phytoplankton populations are very high and the surface waters
are dominated either by dinoflagellates (Fig. 1.10)
or colonial blue-green algae. At this point in the
seasonal cycle, the surface water concentration of
phosphorus is very low and dinoflagellates such
as Ceratium survive by diurnal migration into the
lower part of the lake – where phosphorus levels
are higher. Dinoflagellates are adapted to this daily
migratory activity by their strong swimming motion – which is coupled to an efficient phototactic
capacity.
28
1 INTRODUCTION TO FRESHWATER ALGAE
freshwater heterotrophic dinoflagellate Peridiniopsis
berolinensis uses a fine cytoplasmic filament to make
contact with suitable prey such as insect larvae, then
ingests the contents.
Heterotrophic dinoflagellates are particularly
adapted to conditions where photosynthesis is limiting. A number of colourless species, for example,
are found under the ice of frozen lakes during the
winter season, where photosynthesis is severely limited by low irradiance levels – which are about 1% of
surface insolation.
200µm
1.8 Cryptomonads
Figure 1.10 Autumn dinoflagellate bloom in a eutrophic lake. SEM view of epilimnion phytoplankton
sample (September), showing almost complete dominance by Ceratium hirundinella. See also Fig. 2.8 (seasonal cycle) and Fig. 4.56 (live Ceratium). Photo: Sigee
and Dean.
r An overwintering phase, where dinoflagellates survive on the sediments as resistant cysts. These
non-flagellate cells (Fig. 2.7) lack an equatorial
groove and initially form in surface waters at the
end of the summer/autumn bloom before sinking to the bottom of the lake. Germination of
cysts occurs in early summer, either on sediments or recently mixed waters, giving rise to
vegetative cells which take up newly available
phosphorus.
Ceratium and Peridinium are geographically
widely dispersed, occurring particularly in waters that
have high calcium-ion concentrations (hard waters)
and low levels of inorganic nutrients (see above). Although these two organisms are phototrophic, obtaining their nutrients in inorganic form, other dinoflagellates exhibit some degree of heterotrophy. Some
are able to ingest food particles by engulfment of
whole cells, formation of a feeding veil (pallium)
or use of a feeding tube (phagopod). In some cases,
these dinoflagellates are pigmented and mixotrophic,
combining heterotrophic nutrition with photosynthesis. Other dinoflagellates such as Peridiniopsis are
obligate heterotrophs and colourless. The common
Cryptomonads (Cryptophyta) are a group of relatively inconspicuous algae that are found in both marine and freshwater environments. They are generally
small to medium-sized unicells, and in many standing
waters are a relatively minor part of the phytoplankton assemblage – both in terms of cell numbers and
biomass.
1.8.1 Cytology
The fine structure of cryptomomads is illustrated by
the light microscope images of Rhodomonas (Fig.
4.52) and Cryptomonas (Fig. 4.53), and by the line
drawing shown in Fig. 1.11. Each cell bears two flagella, which are about the same length as the cell
and slightly unequal. One of these flagella propels
the cell, while the other is stiff and non-motile. The
flagella are inserted near to an anterior ventral depression, the vestibulum. The cells tend to be more
convex on one side than the other and the front end,
where the flagella arise, tends to be obliquely truncated. On this side arises a gullet, which is not always clearly visible using a light microscope. Two
elongate chloroplasts are present in Cryptomonas,
lying along the length of the cell (Fig. 4.53) and
on either side of the midline. The colour of cryptomonad chloroplasts may be blue, blue-green, reddish, olive green, brown or yellow-brown depending upon the accessory pigments present. These
may include phycocyanin, phycoerythrin, carotene
and xanthophylls. A pyrenoid is present and starch
29
1.8 CRYPTOMONADS
symbiont that lead to the establishment of plastids
in cryptomonads.
1.8.2
Comparison with euglenoid algae
Cyptomonads share a number of similarities with
euglenoid algae. In both cases:
r algae are fundamentally biflagellate unicells, with
flagella emerging from an apical depression
r cells are essentially naked, with a rigid covering of
the cell occurring inside the plasmalemma
r species able to produce thick walled cysts that can
survive adverse conditions
r different species occur in a variety of aquatic environments, both freshwater and marine
Figure 1.11 Cryptomonad morphology. Diagram of
a non-photosynthetic cryptomonad with two flagella
emerging from a depression. Reproduced, with permission from Graham and Wilcox (2000).
is stored. Further distinctive cytological features,
include:
r a prominent contractile vacuole
r large ejectile organelles (ejectisomes), which discharge their content into the vestibular cavity and
probably act as a defence response to herbivorous
zooplankton
r the presence of an internal ‘cell wall’ (the periplast)
below the plasmalemma, which is composed of
round, oval, rectangular or hexagonal organic
plates
r a complex flagellar root apparatus
r distinctive
plastids, with the associated presence of a much reduced nucleus – referred to
as a nucleomorph; this phylogenetic survival is
thought to be derived from the original (red algal)
r heterotrophic
(colourless) algae are well represented alongside autotrophic (green) species.
In spite of these similarities, there are a number
of major differences between the algal groups, indicating a distinct phylogenetic origin: Key biochemical differences include the presence of starch (stains
blue-black with iodine) in crypomonads, with photosynthetic pigments which include phycobilins and
show some similarity to red algae. Ultrastructural differences also occur in terms of flagellar ornamentation, and flagellar roots are different from euglenoids.
The environmental biology of cryptomonads is also
quite distinct from euglenoids, indicating a different
ecological role for these organisms. Cryptomonads
are genuinely planktonic algae, widespread amongst
the plankton of a range of water bodies – in contrast
to euglenoids, which tend to occupy mud, sand and
water interfaces.
1.8.3
Biodiversity
Cryptomonads are a relatively small group of algae,
with a total of about 12 genera (van den Hoek et al.,
1995) containing approximately 100 known freshwater species and 100 known marine species.
30
1 INTRODUCTION TO FRESHWATER ALGAE
Table 1.10 Variation in Plastid Number and Coloration in Cryptomonads
Trophic State
Plastid Characteristics*
Autotrophic
Rhodomonas – a single boat-shaped,
red coloured plastid (4.52)
Chroomonas – a single H-shaped
blue-green plastid with a pyrenoid
on the bridge.
Cryptomonas – two plastids, each with
a pyrenoid (4.53)
Heterotropohic Chilomonas – a colourless cell that
contains a plastid (leucoplast) that
lacks pigmentation. No pyrenoid
*Figure numbers in brackets.
The low diversity in terms of number of species
is reflected in the uniformity of morphology. There
are no clear colonial forms, though some species are
able to form irregular masses embedded in mucilage
(palmelloid stage) under adverse conditions. Differences do occur in relation to plastids – including
presence or absence, number and pigmentation (Table 1.10). Variations also occur in other key cytological features such as the presence or absence of nucleomorphs, occurrence of ejectisomes and structure of
the periplast – providing a basis for the classification
of these organisms (Novarino and Lucas, 1995).
1.8.4
Ecology
Cryptomonads are typical of temperate, high latitude
standing waters that are meso- to oligotrophic. They
appear to be particularly prominent in colder waters,
becoming abundant in many lakes in early spring,
when they may commence growth under ice. Algae
such as Rhodomonas may form red blooms when
the ice melts in early spring. In other (Antarctic)
lakes, where ice persists, cryptomonad species such
as Cryptomonas sp. and Chroomonas lacustris may
contribute up to 70% of the phytoplankton biomass,
dominating the algal flora for the whole of the limited
summer period (Spaulding et al., 1994).
Cryptomonads are also typical of the surface waters of temperate lakes during the clearwater phase
of the seasonal cycle, between the diatom spring
bloom and the beginning of the mixed summer
bloom. During this period, when algal populations
are severely predated by zooplankton populations,
the limited competition between algal cells favours
rapidly-growing r-selected organisms such as cryptomonads (Sigee, 2004).
In oligotrophic freshwater lakes, cryptomonads
often form large populations about 15–20 m deep
within the lake, where oxygenated surface waters
interface with the anoxic lower part of the water
column. In this location, these algae form deep
water accumulations of photosynthetic organisms,
referred to as ‘deep-chlorophyll maxima’. Studies on
cultures of Cryptomonas phaseolus and C. undulata
isolated from deep chlorophyll maxima (Gervais,
1997) showed that these organisms had optimal
growth under light-limiting conditions, suggesting
photosynthetic adaptation to low light intensities.
Other factors which may be important in the ability
of these organisms to form large depth populations
include – the ability for heterotrophic nutrition,
relative absence of predation by zooplankton,
access to inorganic nutrients regenerated by benthic
decomposition and tolerance of sulfide generated in
the reducing conditions of the hypolimnion.
The ability of cryptomonads to carry out heterotrophic nutrition, using ammonium and organic
sources as a supply of carbon and nitrogen is ecologically important. Evidence for uptake of particulate organic material comes from direct observation of phagocytosis and has been documented in
both pigmented (e.g. Cryptomonas ovata) and various colourless species (Gillott, 1990). Ultrastructural
studies also provide evidence that many pigmented
cryptomonads are mixotrophic, capable of both autotrophic and heterotrophic nutrition. The blue-green
cryptomonad, Chroomonas pochmanni, has a specialized vacuole for capturing and containing bacterial
cells. Bacteria are drawn into this through a small
pore at the cell surface, and subsequently pass into
digestive vesicles within the cytoplasm.
1.8.5 Cryptomonads as bioindicators
Although they are common in oligo- and mesotrophic
conditions, cryptomonads also occur in eutrophic
lakes (Fig. 2.16) and have limited use as
31
1.9 CHRYSOPHYTES
bioindicators. Individual species do not provide the
contemporary diagnostic range for different habitats
seen in other algal groups, and cryptomonads do not
survive as clearly identifiable remains within lake
sediments.
1.9
Chrysophytes
Chrysophytes (Chrysophyta), commonly referred to
as the ‘golden algae’, are a group of microscopic algae that are most readily recognized by their goldenbrown colour (Figs 1.3 and 4.37). This is due to
the presence of the accessory pigment fucoxanthin within the chloroplasts, masking chlorophylls-a
and -c (Table 1.3). There are around 200 genera
and 1000 species of chrysophytes, present mainly in
freshwater although some may be found in brackish
and salt waters.
1.9.1
Cytology
Distinguishing cytological features (Table 1.3) include the presence of two dissimilar (long and
short) flagella (heterokont condition) in motile organisms, plastids with four outer membranes plus triplet
thylakoids, chrysolaminarin as the main storage product (present in special vacuoles) and cell walls composed of pectin – covered in some species with small
silica spines or scales. Lipid droplets also frequently
occur, and an eyespot may be visible in some species,
associated with the chloroplast. In order to survive
adverse conditions round walled cysts (stomatocysts)
may be produced, with silica scales present in some
species. Although most chrysophyte species are photosynthetic, some may be partially heterotrophic or
even fully phagotrophic (Pfandl et al., 2009). Many
species are small and are important members of the
nanoplankton. They are widely distributed throughout freshwater systems, and are of particular interest
in relation to their morphological diversity, ecology
and as indicator organisms. Details of taxonomy and
ultrastructure are given by Kristiansen (2005).
1.9.2
Morphological diversity
Chrysophytes exhibit considerable diversity in their
general organization, ranging from unicellular to
spherical (e.g. Synura – Fig. 4.37) and branching
(e.g. Dinobryon – Fig. 4.2) colonial types. The great
morphological diversity shown by these organisms
(Table 1.11) follows a similar diversity seen in other
Table 1.11 Range of Body Structure and Form in Chrysophytes
General Structure
Colony Status
Examples*
Flagellate cells (monads)
Simple cell wall
Single cells:
Colonies
Colony
Single cells
Colony
Chromulina,
Ochromonas
Uroglena
Dinobryon (4.2)
Mallomonas
Synura (4.37)
Golden amoebae
Cells surrounded by a jelly
Cells surrounded by solid cell wall
Chrysamoeba
Chrysocapsa
Stichogloea
Simple branched filaments
Sphaeridiothrix
Phaeothamnion
Phaeoplaca
Surrounded by envelope (lorica)
Cells covered in silica scales
Non-flagellate unicells
Motile by pseudopod
Non-motile
Colonies with distinctive
morphology
Two-dimensional cell plates
*Figure numbers in brackets.
32
1 INTRODUCTION TO FRESHWATER ALGAE
algal phyla and includes flagellates, amoeboid forms,
cells in jelly (capsal), filamentous algae and plate-like
(thalloid) organisms. Each of these types, however,
can transform into another phase (e.g. flagellates can
become amoeboid) – so that grouping in relation to
morphology is dependent on the stage of life cycle.
The occurrence of a resistant (stomatocyst) phase (see
above) is also a key part of the life cycle.
1.9.3
Ecology
Chrysophytes are ecologically important in a number
of ways (Kristiansen, 2005).
r In
many ecosystems they play an important role
as primary producers. This is particularly the case
for adverse conditions – such low nutrient and acid
lakes (e.g. Nedbalova et al., 2006).
r They have a versatile nutrition, with many members (e.g. Dinobryon) being mixotrophic. These organisms are able, in addition to photosynthesis, to
obtain their carbon from organic sources – either
by surface absorption of organic compounds or by
phagocytosis of particulate organic matter.
r They can be nuisance algae, giving a fishy smell
to drinking water reservoirs when they reach high
population levels.
Table 1.12 Chrysophyte Species as Bioindicators
Strongly acid,
often humic
conditions
Weakly buffered,
slightly acid
clear water lakes
Alkaline
conditions
Alkaline/saline
conditions
Dinobryon pediforme, Synura
sphagnicola, Mallomonas
paludosa, M. hindonii,
M. canina
Mallomonas hamata, Synura
echinulata, Dinobryon
bavaricum var. vanhoeffenii
Mallomonas punctifera, Synura
uvella
Mallomonas tonsurata,
M. tolerans
croscopy for identification – a technique not routinely
used as a tool in water quality assessment.
Although chrysophytes have traditionally been
held to indicate oligotrophic conditions, the situation is more complex. Studies by Kristiansen (2005)
have shown that the presence of a few species such as
Uroglena or Dinobryon may indicate oligotrophy, but
greater chrysophyte species diversity at lower overall
biomass is more indicative of eutrophic conditions.
The value of individual chrysophyte species as ecological indicators varies considerably (Kristiansen,
2005). Synura petersenii, with a broad ecological
range, is of limited use – but other narrow-range
species (Table 1.12) can be used particularly in relation to pH and salinity.
Sediment analysis
1.9.4
Chrysophytes as bioindicators
Their diverse ecological preferences (Table 1.12)
make chrysophyte species potentially useful as environmental indicator organisms – both in terms of contemporary water assessment and sediment analysis.
Contemporary water quality
Although potentially very useful, chrysophytes are
rarely used in monitoring projects. The main reason
for this is that the best ecologically-studied species
(with silica scales) require electron scanning mi-
The main application of chrysophytes as environmental bioindicators is in paleoecology. Their major advantage for this (as with diatoms) is the presence of non-degradable silica cell wall material, so
that they remain undamaged in the lake sediments.
The use of these algae in paleoecology depends on
accurate means of classification and identification,
undisturbed sediment layers and absolute dating of
the layers – e.g. by the lead isotope Pb210 .
Two main types of chrysophyte remains are useful
in freshwater sediments – silica scales (from vegetative planktonic cells) and stomatocysts (resistant
spores). Stomatocysts have the advantage of better
preservation compared to scales, but the disadvantage
33
1.10 DIATOMS
that (with a few exceptions) they cannot be referred to
particular species. Although stomatocysts cannot be
identified in terms of species, they can be recorded as
distinct morphological types (morphotypes). These
can be correlated with other microfossils (e.g. diatoms) and with pollen to obtain environmental indicator values. Quantitative assessment of past aquatic
environments in relation to fossil chrysophytes (stomatocysts) parallels that of diatoms (Section 3.2.2) and
can be obtained in relation to species ratios (environmental indices), transfer functions (Facher &
Schmidt, 1996) and group analysis.
Species ratios The ratio of chrysophytes to other
algal groups may provide a useful index to assess environmental change. Studies by Smol (1985) on the
surface sediments of Sunfish Lake (Canada) demonstrated a marked increase in the ratio of diatoms to
somatocysts, coincident with the arrival of settlers to
the lake catchment area. The diatom/somatocyst ratio
is a trophic index, signalling an increase in eutrophic
status of the lake at the time of human settlement.
The above ratio change resulted from a population
decrease in the chrysophyte Mallomonas (indicating
oligotrophic conditions) and an increase in eutrophic
diatoms. The increase in trophic ratio parallels an increase in the pollen count of ragweed, an indicator of
forest clearance and farming activity.
Transfer functions The environmental properties of stomatocyst morphotypes can be assigned as
numerical descriptors or ‘transfer functions.’ These
define each morphotype in relation to a range of environmental data (pH, conductivity, phosphorus contents and maximal depth of the lake).
The transfer functions have been derived by multivariate analysis of sediment calibration sets, obtained from a series of North American lakes in
which both environmental data and stomatocyst assemblages were available (Charles and Smol, 1994).
showed that differences in pH and lake morphometry
had the greatest influence in determining statistical
associations. PCA analysis separated morphotypes
into three main groups, respectively relating primarily
to acid environments, neutral/alkaline environments
and lake morphometry (e.g. lake depth).
1.10 Diatoms
Diatoms (Bacillariophyta) are a very distinct group
of algae, identifiable under the light microscope by
their yellow-brown coloration (Fig. 1.13) and by the
presence of a thick silica cell wall. This cell wall typically appears highly refractive under the light microscope, giving the cell a well-defined shape. Removal
of surface organic material by chemical oxidation
(Section 2.5.4) reveals a complex cell wall ornamentation – as illustrated in the light microscope images comparing living and digested cells of Stephanodiscus (Fig. 4.58). Cell wall ornamentation can be
seen even more clearly with the higher resolution of
the scanning electron microscope and is important in
species identification.
Diatoms occur as non-flagellate single cells, simple
colonies or chains of cells, and are widely-occurring
in both marine and freshwater environments. Their
success in colonizing and dominating a wide range
of aquatic habitats is matched by their genetic diversity – with a worldwide total of 285 recorded genera,
encompassing 10 000–12 000 species (Round et al.,
1990; Norton et al., 1996). Diatoms are also very
abundant in both planktonic and benthic freshwater
environments, where they can form a large part of the
algal biomass, and are a major contributor to primary
productivity.
1.10.1
Cytology
Distinctive cytological features (Table 1.3) include:
Group analysis The various environmental factors which influence correlations between morphotypes can be identified by statistical analysis – particularly principal component analysis (PCA). Studies
by Duff and Smol (1995) on 181 morphotypes obtained from 71 different lakes (in the United States)
r plastids
with periplasmic endoplasmic reticulum
and the presence of girdle lamellae
r chrysolaminarin
and lipid food reserves, present
outside the plastid
34
1 INTRODUCTION TO FRESHWATER ALGAE
ra
distinctive cell wall, the frustule, composed of
opaline silicon dioxide (silica) together with organic coatings.
Silica cell wall
The presence of silica in the diatom cell wall can be
detected by cold digestion in a strongly oxidizing acid
to remove organic matter (Section 2.54). If the cell
wall resists this treatment it is probably silica. The
cell wall of diatoms differs from that of other algae
in being almost entirely inorganic in composition. It
has evolved as an energy-efficient structure, requiring
significantly less energy to manufacture than the cellulose, protein and mucopeptide cell walls of other
algae (Falkowski and Raven, 1997). This gives diatoms a major ecological advantage at times when
photosynthesis is limited (e.g. early in the seasonal
cycle), but has a number of potential disadvantages.
r The cell wall of diatoms is very dense. These organisms are only able to stay in suspension in
turbulent conditions, limiting the development of
extensive planktonic diatom populations to unstratifiied waters.
r The
formation of the diatom cell wall is dependent on an adequate supply of soluble silica (silicic
acid) in the surrounding water. The diatom spring
bloom of temperate lakes strips out large quantities
of silica from the water, reducing concentrations to
a level that becomes limiting.
r Unlike other types of cell wall material, silica is
rigid and unable to expand. This means that daughter cells are unable to enlarge and progressive cell
divisions result in a gradual decrease in cell size.
Ultimately this decrease reaches a critical level,
at which point sexual reproduction is required to
completely shed the original cell wall and form
new, large daughter cells.
Cell wall structure
The frustule is composed of two distinct halves – the
epitheca and hypotheca – which fit together rather
like the lid and base of a pill-box (Fig. 1.12). At
cell division, two new walls are formed internally
at the cell equator ‘back to back’, and become the
hypothecae of the two daughter cells. The original
epitheca and hypotheca of the parent diatom become
1
B
2
C
e
e
3
frustule
structure – comparison of centric
and pennate diatoms. Centric
diatom: A. Separate views of
epitheca (e) 1. Epivalve 2. Mantle
of epivalve. 3. Epicingulum and
hypotheca (h) 4. Hypocingulum. 5.
Hypovalve. B. Complete frustule –
girdle or side view (showing overlap
of cingula). C. Valve or face view
of epivalve. Pennate diatom: D.
Complete frustule – girdle view.
E. Valve view, showing apical or
longitudinal axis (aa) and transapical
or transverse axis (ta).
h
A
Figure 1.12 Diatom
4
h
aa
5
e
h
ta
D
ta
E
aa
35
1.10 DIATOMS
the epithecae of the two daughter cells, so the epitheca
is always the oldest part of the frustule.
In both centric and pennate diatoms (see below)
the epitheca consists of two main parts – a circular disc (the valve) with a rim (the mantle) plus
an attached band or girdle (the epicingulum). The
hypotheca correspondingly consists of a hypovalve,
hypovalve mantle and hypocingulum. When the epitheca and hypotheca fit together to form the complete
frustule the hypocingulum fits inside the epicingulum
to give an overlap.
Because of frustule morphology (Fig. 1.12), diatoms can be observed from either a girdle (side)
view or from a valve (face) aspect – with girdle and
valve views of live diatoms being clearly distinguishable under the light microscope (Fig. 1.13). The two
possible valve views, of the top (epivalve) or bottom
(hypovalve) of the diatom, are not distinguishable in
many genera – but do differ in some cases (e.g. Cocconeis, Achnanthes). Axes of symmetry in pennate
diatoms are shown in Figs 1.12 and 1.13.
presence of one major surface structure – the raphe
– is clearly associated with locomotion. The secretion of mucus from this channel or canal promotes
movement on solid surfaces. In some diatoms such
as Nitzschia (Fig. 4.70a, b) the raphe is elevated from
the main diatom surface as a keel, allowing more intimate contact between the raphe and substrata. Such
keeled diatoms are able to move particularly well on
fine sediments, and reach their maximum abundance
in the epipelon of pools and slowly-flowing streams
(Lowe, 2003). A raphe is not always present in pennate diatoms, and is never seen in centric diatoms
(see below).
1.10.2
Morphological diversity
The morphological diversity of diatoms can be considered in relation to two main aspects – the distinction between centric/pennate diatoms and the range
of unicellular to colonial forms.
Centric and pennate diatoms
Frustule markings
A wide range of surface markings can be seen on
the face (epivalve and hypovalve) of diatoms. These
have been recorded in considerable detail (Barber and
Haworth, 1981; Round et al., 1990; Wehr and Sheath,
2003) and form the basis for the classification and
identification of these organisms (Chapter 4).
Clear visualization of the frustule markings requires the high resolution of either oil immersion
(light microscopy) or scanning electron microscopy,
and is normally carried out after the removal of
surface organic matter by chemical (acid digestion)
cleaning. Chemical fixation for scanning electron microscopy may also strip away some of the overlying
organic material to reveal frustule surface structure.
The terminology of diatom morphology (see
Glossary) includes various descriptors of frustule
markings – including eye-shaped structures (ocelli),
small pores (punctae) and fine lines (striae). Illustrations of diatom surface markings are shown diagrammatically in Fig. 1.14 and in various figures and
plates in Chapter 4. Although the biological significance of much of this surface detail is obscure, the
Diatoms can be separated into two major groupings – centric and pennate diatoms, based primarily
on cell shape and frustule morphology (Fig. 1.12).
Centric diatoms typically have a discoid or cyclindrical shape, with a radial symmetry when seen in face or
‘valve’ view. Pennate diatoms have an elongate bilateral symmetry, with longitudinal and transverse axes,
and often have the appearance of a ‘feathery’ (hence
‘pennate’) ornamentation when seen in surface view
(Fig. 1.13). Pennate diatoms have a range of shapes
(Barber and Haworth, 1981), with broad division
(Fig. 1.15) into isopolar (ends of valve similar in size
and shape), heteropolar (ends of valve differing in size
and shape), asymmetrical (outline dissimilar either
side of the longitudinal axis) and symmetrical (outline
of valve similar either side of the longitudinal axis).
The fundamental differences between centric and
pennate diatoms in relation to structure and symmetry reflect a range of other distinguishing features
(Table 1.13) – including motility, number and size
of plastids, sexual reproduction and ecology. Centric diatoms are mainly planktonic algae, and the
occurrence of oogamy – with the production of a
A
15µm
B
C
Figure 1.13 Details of frustule structure in a pennate diatom (Pinnularia). Top: Diagrams of cell wall structure, with
principal axes. Left: Valve view. aa – apical axis, ta – transapical axis, pn – polar nodule, r – raphe. Middle: Girdle view.
pa – pervalvular axis, vp – valvar plane, h – hypotheca, e – epitheca, g – girdle. Right: Transverse section. Reproduced,
with permission, from Fritsch (1956). Bottom: Light microscope images of fresh (unfixed) cells in valve (B) and girdle
(C) view.
1.10 DIATOMS
37
adaptation to more restricted benthic environments.
Fossil and molecular evidence suggests that centric
diatoms arose prior to pennate ones, implying that in
these algae isogamy has been phylogenetically derived from oogamy (Edlund and Stoermer, 1997).
This is in contrast to the evolutionary sequence normally accepted for other algal groups - particularly
green and brown algae.
Unicellular and colonial diatoms
The genetic diversity of diatoms is not matched by
the complexity of their morphological associations –
which are limited to small chains and groups of cells.
In planktonic diatoms, simple filamentous (e.g. Aulacoseira – Fig. 4.9) and radial (e.g. Asterionella –
Figs 1.16, 4.42) colonies have evolved, but there is
no formation of spherical globular colonies that are a
key feature of some other algal groups.
Colonial forms of diatoms arise by means of simple
linkages, which are of three main types.
r Mucilage
Figure 1.14 Diatom valve markings. 1 internal septa;
2 transapical costae; 3 raphe in thickened ribs; 4 normal
raphe; 5 shortened raphe in ribs; 6 parallel striae; 7 radiate
striae; 8 central area round; 9 central area transverse;
10 central area small; 11 central area acute angled; 12
punctae in radial rows interspersed with subradial rows;
13 coarse areolae; 14 areolae grouped in segments; 15
sections of valve alternately raised and depressed giving
shaded appearance; 16 central region raised or depressed;
17 pennate with raised or depressed areas; 18 ocelli on
surface; 19 rudimentary raphe as in Eunotia; 20 raphe
curved to point at centre as in Epithemia. Reproduced,
with permission, from Bellinger, 1992.
pads. These occur at the ends of cells
to join diatom cells in various ways. Asterionella
has elongate cells which are joined at their inner
ends to form stellate colonies. In Tabellaria, cells
are linked valve to valve as short stacks, with connection of the stacks at frustule edges by mucilage
pads to form a zig-zag pattern. Mucilage pads are
seen particularly clearly in Fig. 2.28, where chains
of Tabellaria are attached to Cladophora.
r Interlinking spines. In diatoms such as Aulacoseira,
chains of cells are joined valve to valve by spines.
The valves are of two main types (Davey and Crawford, 1986)
– Separation valves, with long tapering spines and
straight rows of pores,
large number motile sperm, is regarded as a strategy
for increasing the efficiency of fertilization in open
water environments. Pennate diatoms are almost always isogamous, with equal sized non-flagellate gametes (two per parent cell). In these algae, efficiency
of fertilization is promoted by the pairing of gamete
parental cells prior to gamete formation, a possible
– Linking valves, with short spines and curved rows
of pores
Cells are strongly linked into chains via the short
spines on the linking valves, but weakly linked via
the long spines of the separation valves – where
38
1 INTRODUCTION TO FRESHWATER ALGAE
Figure 1.15 Diatom morphology.
1 pennate diatom with raphe; 2
with pseudoraphe; 3 sulcus (s) on
Melosira; 4 spines on margin of
Stephanodiscus; 5 valve shapes: (a)
isopolar, (b) heteropolar, (c) asymmetrical, (d) symmetrical.
Reproduced, with permission, from
Bellinger, 1992.
filament breakage easily occurs (Fig. 4.9). The
length of filaments within a population of Aulacoseira depends on the proportion of separation
valves to linking valves.
r Gelatinous
stalks. Biofilm diatoms such as
Rhoicosphenia (Fig. 4.67) and Gomphonema (Fig.
2.29) have gelatinous stalks, which attach the diatom to the substratum and join small groups of
cells together as a single colony. In these diatoms,
the stalk material is secreted via apical pores. Some
species of Cymbella also produce a gelatinous tubelike filament.
1.10.3 Ecology
Diatoms are ubiquitous in both standing and running
freshwaters – occurring as planktonic, benthic, epiphytic (on higher plants and other algae) and epizoic
(on animals, such as zooplankton) organisms. They
are equally successful as free floating and attached
Table 1.13 Centric and Pennate Diatoms
Characteristic
Centric Diatoms
Pennate Diatoms
Symmetry
Examples*
Radial
Stephanodiscus (4.58)
Cyclotella (4.62)
Non-motile
Bilateral
Pinnularia (1.13, 4.71)
Navicula (4.73)
Some (raphid) diatoms are actively motile
Many discoid plastids
Two large plate-like plastids
Independent formation of gametes from parent
cells
Oogamous – production of one or two eggs per
parent cell
4–128 sperm per parent cell. Each with a single
flagellum bearing two rows of mastigonemes
Mainly planktonic, typical of open water
Pairing of parental cells prior to gamete
production
Isogamous
Gliding motility
Plastids
Sexual reproduction†
Initiation
Egg cells
Sperm cells
Ecology
*Figure numbers in brackets.
and Syvertsen, 1997.
† Hasle
Few amoeboid, non-flagellate sperm cells
Planktonic, epiphytic and benthic forms
39
1.10 DIATOMS
constituents within the mixed phytoplankton population over much of the annual cycle – and are regarded
as holoplanktonic.
Attached diatoms: biofilm formation
Au
d
A
100µm
Figure 1.16 Spring diatom bloom in a eutrophic
lake. SEM view of lake surface phytoplankton sample
(March), showing numerous stellate colonies of Asterionella (A) with some filamentous Aulacoseira (Au). The
large amount of debris (d) occurs due to the complete
mixing of the water column at this time of year. See also
Figs 2.8 (seasonal cycle), 4.8 (live Aulacoseira) and 4.42
(live Asterionella). Photo: Sigee and Dean.
forms, where they may be respectively important in
plankton bloom formation and biofilm development.
Diatoms are major components of biofilms, where
they are present early in the colonization sequence
and also within the mature periphyton community.
See later for examples of diatoms in river biofilm
(Fig. 2.23), reed biofilm (Fig. 2.29) and as attached
epiphytes (Fig. 2.28).
1.10.4
Diatoms as bioindicators
The major use of diatoms as bioindicators of water
quality (both lotic and lentic systems) is detailed in
Chapter 3. These algae can be isolated from existing
live populations (contemporary analysis) or from sediments (fossil diatoms) and quantitative information
on water quality from species counts can be obtained
from taxonomic indices, multivariate analysis, transfer functions and species assemblage analysis.
Contemporary analysis
Planktonic diatoms: bloom formation
In many temperate lakes, diatoms such as Asterionella and Tabellaria dominate the phytoplankton
population in Spring and early Summer (Fig. 1.16),
at a time when inorganic nutrients (N, P, Si) are at high
concentration, light and temperature levels are rising
and lake turbulence is maintained by moderate wind
action. At this point in the seasonal cycle, diatoms are
able to out-compete other micro-algae due to their tolerance of low temperature and low light conditions,
coupled with their ability to grow in turbulent water. Autumn blooms of Fragilaria and Asterionella
are also a common feature of many lakes, with the
diatoms part of a mixed phytoplankton population.
Diatoms such as Aulacoseira italica are regarded
as meroplanktonic, with major growth in very early
Spring, but spending the rest of the year as resistant cells on lake sediments. Other diatoms, such as
Asterionella and Tabellaria also form major blooms
in Spring, but are additionally present as minor
Many diatom species have distinct ecological preferences and tolerances, making them useful indicators
of contemporary ecological conditions. These preferences relate to degree of water turbulence, inorganic
nutrient concentrations, organic pollution, salinity,
pH and tolerance of heavy metals. Round (1993) also
lists a number of other advantages in using diatoms
as indicators – including easy field sampling, high
sensitivity to water quality but relative insensitivity
to physical parameters in the environment and easy
cell counts. Diatoms are also the most diverse group
of algae present in freshwaters, making them the
most ideal assemblage for calculation of bioindices
(Section 3.4.5).
Diatoms, once cleaned and mounted for identification, make excellent permanent slides which can
form an important historical record for a location.
Although there are hundreds of diatom species that
may be present only dominant species need to be
used in an assessment. Benthic diatoms have been
40
1 INTRODUCTION TO FRESHWATER ALGAE
particularly useful in assessing water quality of rivers,
and the wide range of indices that have been devised
is discussed in Chapter 3.
Lake sediments
Diatoms, more than any other group of algae, are used
to monitor historical ecological conditions from sediment analysis – as discussed in Section 3.2.2. The
major role of diatoms in this lies in the resistance of
the diatom frustule to biodegradation, coupled with
ease of identification from frustule morphology and
the wide range of species with clear ecological preferences see in freshwater environments.
The importance of diatoms in environmental analyses is indicated by the European Diatom Database
Initiative (EDDI). This key web-based site includes
electronic images and data handling software, and is
particularly useful for lake sediment analysis (Section 3.2.2). Other databases, such as the stream benthic database of Gosselain et al. (2005), are more
applicable to contemporary environmental analysis.
visible to the naked eye when occurring in reasonable
numbers (Sheath and Hambrook, 1990). The range of
morphologies include gelatinous filaments (e.g. Batrachospermum), pseudoparenchymatous forms (e.g.
Lemanea) and a flat thallus of tiered cells (Hildenbandia). Many of these shapes can be an advantage in
resisting the forces exerted in swiftly flowing waters.
Although freshwater red algae (such as the large filamentous alga Batrachospermum) are largely found
in streams and rivers, Rhodophyta may also occur
as marine invaders of lakes and brackish environments. Certain freshwater red algae in the littoral
zones of the Great Lakes Basin (United States), for
example, appear to be originally marine and to have
lost the capacity for sexual reproduction. These include the filamentous red alga Bangia atropurpurea
(Lin and Blum, 1977), which reproduces only by
asexual monospores – in contrast to marine species
which undergo alternation of generations and carry
out sexual reproduction. Attached red algae (e.g.
Chroodactylon ramosum) also contribute to the epiphytic flora of lake periphyton.
1.12 Brown algae
1.11 Red algae
Red algae (Rhodophyta) are predominantly marine in distribution, with only 3% of over 5000
species worldwide occurring in true freshwater habitats (Wehr and Sheath, 2003). Although termed redalgae, the level of accessory pigments (phycoerythrin and phycocyanin) may not be sufficient to mask
the chlorophyll – resulting in an olive-green to blue
(rather than red) coloration. The relatively common
river alga Batrachospermum, for example, typically
appears bluish-green in colour (Fig. 4.4) – giving no
clue to its rhodophyte affinity.
Major cytological features of red algae (Table 1.3)
include the absence of flagellae, presence of floridean
starch as the major food reserve, characteristic photosynthetic pigments (chlorophyll-a only, presence
of phycobilins) and distinctive plastid structure
(unstacked thylakoids, no external endoplasmic
reticulum).
Many of the freshwater species (which occur
mainly in streams and rivers) are quite large, being
As with red algae, brown algae (Phaeophyta) are almost entirely marine – with less than 1% of species
present in freshwater habitats (Wehr and Sheath,
2003). These species are entirely benthic, either in
lakes or rivers, and have a very scattered distribution.
Cytological features of this group (Table 1.3) include heterokont flagella (reproductive cells only),
presence of laminarin as the major food reserve, characteristic photosynthetic pigments (chlorophyll-a and
-c, β and ε carotenes) and distinctive plastid structure (triple thylakoids, enclosing endoplasmic reticulum). Freshwater brown algae include genera such
Pleurocladia and Heribaudiella, and are the least
diverse of all freshwater algae. Their morphologies
are based on a relatively simple filamentous structure
(forming mats or crusts), and they lack the complex
macro-morphology typical of the brown seaweeds.
The freshwater brown-algae have not been fully studied and their ecological characteristics are not well
known. All are benthic and may be recognized by
their possession of large sporangia (Wehr, 2003).